The trace tests aren't strictly for performance. Since we'll be using
them for correctenss testing as well it makes sense to move them out
of the perf_tests/ folder.

Also renames RestrictedTraceTests.md to README.md so it'll load
automatically in gitiles.

Bug: angleproject:4090
Bug: b/168049670
Change-Id: I8be9f1d831489a9abf534d049a93441687850142
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2416913
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>