tests: define assert_empty for Ruby <= 1.8

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/test/testing_env.rb

@@ -88,3 +88,10 @@ module VersionAssertions
     assert_nil Version.parse(url)
   end
 end
+
+module Test::Unit::Assertions
+  def assert_empty(obj, msg=nil)
+    assert_respond_to(obj, :empty?, msg)
+    assert(obj.empty?, msg)
+  end if RUBY_VERSION.to_f <= 1.8
+end