dEQP test_expectations parser should allow nested overrides.

When an expectation line is not used for any test, it's likely
duplicated or misspelled, or the test was removed. The parser should
output these errors to make it easier to keep the expectations up to
date.

Also necessary to allow longer/more specific expectations to override
shorter ones, so the scenario where you FAIL one set of tests across
platforms, and SKIP a subset of that group on one platform will still be
allowed.

Bug: angleproject:3368
Change-Id: If0470cb6fb12f8d3daa97df0c701185fa086668c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1564725
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>