linkage: fix --test exit code.

Ensure that a non-zero exit code is set both for missing random dylibs
and random missing dependencies.

Additionally, while we are here, drastically trim down the public
interface for this class to the bare minimum and allow getting the
output from `display_test_output` as a variable.

Fixes issue mentioned by @ilovezfs in:
https://github.com/Homebrew/brew/pull/3940#issuecomment-383794520

Library/Homebrew/dev-cmd/linkage.rb

@@ -21,7 +21,7 @@ module Homebrew
       result = LinkageChecker.new(keg)
       if ARGV.include?("--test")
         result.display_test_output
-        Homebrew.failed = true if result.broken_dylibs?
+        Homebrew.failed = true if result.broken_library_linkage?
       elsif ARGV.include?("--reverse")
         result.display_reverse_output
       else

Library/Homebrew/extend/os/mac/formula_cellar_checks.rb

@@ -66,10 +66,10 @@ module FormulaCellarChecks
     keg = Keg.new(formula.prefix)
     checker = LinkageChecker.new(keg, formula)
 
-    return unless checker.broken_dylibs?
+    return unless checker.broken_library_linkage?
     output = <<~EOS
       #{formula} has broken dynamic library links:
-        #{checker.broken_dylibs.to_a * "\n  "}
+        #{checker.display_test_output}
     EOS
     tab = Tab.for_keg(keg)
     if tab.poured_from_bottle

Library/Homebrew/linkage_checker.rb

@@ -2,10 +2,6 @@ require "keg"
 require "formula"
 
 class LinkageChecker
-  attr_reader :keg, :formula
-  attr_reader :brewed_dylibs, :system_dylibs, :broken_dylibs, :broken_deps, :variable_dylibs
-  attr_reader :undeclared_deps, :unnecessary_deps, :reverse_links
-
   def initialize(keg, formula = nil)
     @keg = keg
     @formula = formula || resolve_formula(keg)
@@ -21,6 +17,44 @@ class LinkageChecker
     check_dylibs
   end
 
+  def display_normal_output
+    display_items "System libraries", @system_dylibs
+    display_items "Homebrew libraries", @brewed_dylibs
+    display_items "Indirect dependencies with linkage", @indirect_deps
+    display_items "Variable-referenced libraries", @variable_dylibs
+    display_items "Missing libraries", @broken_dylibs
+    display_items "Broken dependencies", @broken_deps
+    display_items "Undeclared dependencies with linkage", @undeclared_deps
+    display_items "Dependencies with no linkage", @unnecessary_deps
+  end
+
+  def display_reverse_output
+    return if @reverse_links.empty?
+    sorted = @reverse_links.sort
+    sorted.each do |dylib, files|
+      puts dylib
+      files.each do |f|
+        unprefixed = f.to_s.strip_prefix "#{@keg}/"
+        puts "  #{unprefixed}"
+      end
+      puts unless dylib == sorted.last[0]
+    end
+  end
+
+  def display_test_output(puts_output: true)
+    display_items "Missing libraries", @broken_dylibs, puts_output: puts_output
+    display_items "Broken dependencies", @broken_deps, puts_output: puts_output
+    puts "No broken library linkage" unless broken_library_linkage?
+  end
+
+  def broken_library_linkage?
+    !@broken_dylibs.empty? || !@broken_deps.empty?
+  end
+
+  private
+
+  attr_reader :keg, :formula
+
   def dylib_to_dep(dylib)
     dylib =~ %r{#{Regexp.escape(HOMEBREW_PREFIX)}/(opt|Cellar)/([\w+-.@]+)/}
     Regexp.last_match(2)
@@ -113,51 +147,6 @@ class LinkageChecker
     end
   end
 
-  def display_normal_output
-    display_items "System libraries", @system_dylibs
-    display_items "Homebrew libraries", @brewed_dylibs
-    display_items "Indirect dependencies with linkage", @indirect_deps
-    display_items "Variable-referenced libraries", @variable_dylibs
-    display_items "Missing libraries", @broken_dylibs
-    display_items "Broken dependencies", @broken_deps
-    display_items "Undeclared dependencies with linkage", @undeclared_deps
-    display_items "Dependencies with no linkage", @unnecessary_deps
-  end
-
-  def display_reverse_output
-    return if @reverse_links.empty?
-    sorted = @reverse_links.sort
-    sorted.each do |dylib, files|
-      puts dylib
-      files.each do |f|
-        unprefixed = f.to_s.strip_prefix "#{@keg}/"
-        puts "  #{unprefixed}"
-      end
-      puts unless dylib == sorted.last[0]
-    end
-  end
-
-  def display_test_output
-    display_items "Missing libraries", @broken_dylibs
-    display_items "Broken dependencies", @broken_deps
-    display_items "Dependencies with no linkage", @unnecessary_deps
-    puts "No broken dylib links" if @broken_dylibs.empty?
-  end
-
-  def broken_dylibs?
-    !@broken_dylibs.empty?
-  end
-
-  def undeclared_deps?
-    !@undeclared_deps.empty?
-  end
-
-  def unnecessary_deps?
-    !@unnecessary_deps.empty?
-  end
-
-  private
-
   # Whether or not dylib is a harmless broken link, meaning that it's
   # okay to skip (and not report) as broken.
   def harmless_broken_link?(dylib)
@@ -171,20 +160,22 @@ class LinkageChecker
 
   # Display a list of things.
   # Things may either be an array, or a hash of (label -> array)
-  def display_items(label, things)
+  def display_items(label, things, puts_output: true)
     return if things.empty?
-    puts "#{label}:"
+    output = "#{label}:"
     if things.is_a? Hash
       things.keys.sort.each do |list_label|
         things[list_label].sort.each do |item|
-          puts "  #{item} (#{list_label})"
+          output += "\n  #{item} (#{list_label})"
         end
       end
     else
       things.sort.each do |item|
-        puts "  #{item}"
+        output += "\n  #{item}"
       end
     end
+    puts output if puts_output
+    output
   end
 
   def resolve_formula(keg)