ASAN builds can detect OOB memory accesses. This is very useful for
diagnosing some kinds of flaky OOB crashes.

Removes a test that was calling BufferSubData with a null data
argument. This behaviour is undefined and we shouldn't be testing it.

Also includes some minor refactorings and script changes.

Bug: angleproject:5133
Change-Id: I5a7d9a5500c50a51f6369e944a5f0a533709f00d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3002510Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>