Fix HOMEBREW_BREW_FILE for unit tests.

Library/Homebrew/global.rb

@@ -39,7 +39,7 @@ else
 end
 
 if not defined? HOMEBREW_BREW_FILE
-  HOMEBREW_BREW_FILE = ENV['HOMEBREW_BREW_FILE'] or `which brew`.chomp
+  HOMEBREW_BREW_FILE = ENV['HOMEBREW_BREW_FILE'] || `which brew`.chomp
 end
 
 HOMEBREW_PREFIX = Pathname.new(HOMEBREW_BREW_FILE).dirname.parent # Where we link under