Bug 987487 - [Gaia] [Build] Refactoring webapp-optimize.js and unit tests

https://bugzilla.mozilla.org/show_bug.cgi?id=987487
Thing is that we don't remove `tests` but only `test`.
And by using `/^shared/` here instead of `/^(shared|tests?)$/` we are trying to optimize at least one test file `contacts/test/unit/mock_contacts_index.html`.
I think we should keep blacklisting `tests` until we figure out why we copy/keep tests in build-stage.
(By the way clean-build-files is weird, instead of deleting files from build-stage, I would have either prevented copying files in build-stage in first place -or- just ignore them when creating zip packages)