tests: add generic flag. (#212)

Makes it easier to quickly do tests of the generic/cross-platform
backend.

Library/Homebrew/cmd/tests.rb

@@ -6,6 +6,7 @@ module Homebrew
       ENV["HOMEBREW_NO_ANALYTICS_THIS_RUN"] = "1"
       ENV["TESTOPTS"] = "-v" if ARGV.verbose?
       ENV["HOMEBREW_NO_COMPAT"] = "1" if ARGV.include? "--no-compat"
+      ENV["HOMEBREW_TEST_GENERIC_OS"] = "1" if ARGV.include? "--generic"
       if ARGV.include? "--coverage"
         ENV["HOMEBREW_TESTS_COVERAGE"] = "1"
         FileUtils.rm_f "coverage/.resultset.json"