Make Test::Unit assertions available in formula tests

Closes Homebrew/homebrew#18753.
Closes Homebrew/homebrew#20358.

Library/Homebrew/cmd/test.rb

@@ -2,6 +2,7 @@ require 'extend/ENV'
 require 'hardware'
 require 'keg'
 require 'timeout'
+require 'test/unit/assertions'
 
 module Homebrew extend self
   TEST_TIMEOUT_SECONDS = 5*60
@@ -31,6 +32,9 @@ module Homebrew extend self
         Timeout::timeout TEST_TIMEOUT_SECONDS do
           raise if f.test == false
         end
+      rescue Test::Unit::AssertionFailedError => e
+        ofail "#{f.name}: failed"
+        puts e.message
       rescue Exception
         ofail "#{f.name}: failed"
       end

Library/Homebrew/formula.rb

@@ -562,6 +562,8 @@ class Formula
   end
 
   def test
+    require 'test/unit/assertions'
+    extend(Test::Unit::Assertions)
     ret = nil
     mktemp do
       @testpath = Pathname.pwd