Let the test environment inject its own load path

Library/Homebrew/cmd/postinstall.rb

@@ -9,7 +9,7 @@ module Homebrew
     args = %W[
       nice #{RUBY_PATH}
       -W0
-      -I #{HOMEBREW_LIBRARY_PATH}
+      -I #{HOMEBREW_LOAD_PATH}
       --
       #{HOMEBREW_LIBRARY_PATH}/postinstall.rb
       #{formula.path}

Library/Homebrew/cmd/test.rb

@@ -29,7 +29,7 @@ module Homebrew
         args = %W[
           #{RUBY_PATH}
           -W0
-          -I #{HOMEBREW_LIBRARY_PATH}
+          -I #{HOMEBREW_LOAD_PATH}
           --
           #{HOMEBREW_LIBRARY_PATH}/test.rb
           #{f.path}

Library/Homebrew/config.rb

@@ -50,3 +50,9 @@ end
 HOMEBREW_LOGS = Pathname.new(ENV['HOMEBREW_LOGS'] || '~/Library/Logs/Homebrew/').expand_path
 
 HOMEBREW_TEMP = Pathname.new(ENV.fetch('HOMEBREW_TEMP', '/tmp'))
+
+if not defined? HOMEBREW_LIBRARY_PATH
+  HOMEBREW_LIBRARY_PATH = Pathname.new(__FILE__).realpath.parent.join("Homebrew")
+end
+
+HOMEBREW_LOAD_PATH = HOMEBREW_LIBRARY_PATH

Library/Homebrew/formula_installer.rb

@@ -472,7 +472,7 @@ class FormulaInstaller
     args = %W[
       nice #{RUBY_PATH}
       -W0
-      -I #{HOMEBREW_LIBRARY_PATH}
+      -I #{HOMEBREW_LOAD_PATH}
       --
       #{HOMEBREW_LIBRARY_PATH}/build.rb
       #{formula.path}

Library/Homebrew/test/lib/config.rb

@@ -11,6 +11,8 @@ TEST_TMPDIR = ENV.fetch("HOMEBREW_TEST_TMPDIR") { |k|
 HOMEBREW_PREFIX        = Pathname.new(TEST_TMPDIR).join("prefix")
 HOMEBREW_REPOSITORY    = HOMEBREW_PREFIX
 HOMEBREW_LIBRARY       = HOMEBREW_REPOSITORY+"Library"
+HOMEBREW_LIBRARY_PATH  = File.expand_path("../../..", __FILE__)
+HOMEBREW_LOAD_PATH     = [File.expand_path("..", __FILE__), HOMEBREW_LIBRARY_PATH].join(":")
 HOMEBREW_CACHE         = HOMEBREW_PREFIX.parent+"cache"
 HOMEBREW_CACHE_FORMULA = HOMEBREW_PREFIX.parent+"formula_cache"
 HOMEBREW_CELLAR        = HOMEBREW_PREFIX.parent+"cellar"