Improve use of default_prefix? in tests.

Library/Homebrew/os/linux/global.rb

@@ -1,5 +1,5 @@
 module Homebrew
-  DEFAULT_PREFIX = if ENV["HOMEBREW_FORCE_HOMEBREW_ON_LINUX"]
+  DEFAULT_PREFIX ||= if ENV["HOMEBREW_FORCE_HOMEBREW_ON_LINUX"]
     "/usr/local".freeze
   else
     "/home/linuxbrew/.linuxbrew".freeze

Library/Homebrew/test/diagnostic_checks_spec.rb

@@ -164,7 +164,7 @@ describe Homebrew::Diagnostic::Checks do
   end
 
   specify "#check_homebrew_prefix" do
-    # the integration tests are run in a special prefix
+    allow(Homebrew).to receive(:default_prefix?).and_return(false)
     expect(subject.check_homebrew_prefix)
       .to match("Your Homebrew's prefix is not #{Homebrew::DEFAULT_PREFIX}")
   end

Library/Homebrew/test/support/lib/config.rb

@@ -41,3 +41,8 @@ PATCH_B_SHA256 = "57958271bb802a59452d0816e0670d16c8b70bdf6530bcf6f78726489ad89b
 
 TEST_SHA1   = "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef".freeze
 TEST_SHA256 = "deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef".freeze
+
+# For testing's sake always assume the default prefix
+module Homebrew
+  DEFAULT_PREFIX = HOMEBREW_PREFIX.to_s.freeze
+end

Library/Homebrew/test/utils/analytics_spec.rb

@@ -9,19 +9,18 @@ describe Utils::Analytics do
       end
 
       it "returns OS_VERSION and prefix when HOMEBREW_PREFIX is a custom prefix" do
-        stub_const("HOMEBREW_PREFIX", "blah")
+        allow(Homebrew).to receive(:default_prefix?).and_return(false)
         expect(described_class.os_prefix_ci).to include("#{OS_VERSION}, #{described_class.custom_prefix_label}")
       end
 
+      it "does not include prefix when HOMEBREW_PREFIX is the default prefix" do
+        expect(described_class.os_prefix_ci).not_to include(described_class.custom_prefix_label)
+      end
+
       it "includes CI when ENV['CI'] is set" do
         ENV["CI"] = "true"
         expect(described_class.os_prefix_ci).to include("CI")
       end
-
-      it "does not include prefix when HOMEBREW_PREFIX is the default prefix" do
-        stub_const("HOMEBREW_PREFIX", Homebrew::DEFAULT_PREFIX)
-        expect(described_class.os_prefix_ci).not_to include(described_class.custom_prefix_label)
-      end
     end
   end
 

Library/Homebrew/utils/analytics.rb

@@ -16,7 +16,7 @@ module Utils
       def os_prefix_ci
         @os_prefix_ci ||= begin
           os = OS_VERSION
-          prefix = ", #{custom_prefix_label}" if Homebrew.default_prefix?
+          prefix = ", #{custom_prefix_label}" unless Homebrew.default_prefix?
           ci = ", CI" if ENV["CI"]
           "#{os}#{prefix}#{ci}"
         end