Provide a method to access test fixtures

Formulae should not require knowledge of Homebrew's internal directory
structure, and we should be able to alter it without modifying
individual formulae.

Library/Homebrew/formula.rb

@@ -498,6 +498,10 @@ class Formula
   def test
   end
 
+  def test_fixtures(file)
+    HOMEBREW_LIBRARY.join("Homebrew", "test", "fixtures", file)
+  end
+
   protected
 
   # :startdoc: