test-parsing: Add functionality to specify "always true flags".

Many parsing tests need to pin a certain flag, and apart from the pinned flag,
we want to keep the "test with all combinations" behavior for the non-pinned
flags.

R=ulan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/349603004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21900 ce2b1a6d-e550-0410-aec6-3dcde31c8c00