Fix memory leaks in invariant tests. Tests weren't properly destroying the compiler instance. BUG=angleproject:2094 Change-Id: I65eb5a02ba741c6f48f09f03b84ded402581e89f Reviewed-on: https://chromium-review.googlesource.com/550569 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>