Optional Diff tests based on configuration

[OmarElfouly]

Can we please add some plumbing to diff tests to have optional diff tests based on configuration? Honestly it's high time we have this anyway, it's something we've been lacking for a looooongggg time.

I'd work with Anna to make this happen, she rewrote a lot of the diff test infra so should understand how we should do this.

Originally posted by @LalitMaganti in #2375 (comment)

[JoeNotCharlesGoogle]

The Chrome diff tests are currently broken because of this. They run a subset of tests that don't include any Etm tests, but do include StdlibSmoke, which does "INCLUDE *" to make sure the whole stdlib builds. Since the stdlib now includes etm.sql unconditionally, and Chrome builds with enable_perfetto_etm_importer = false, the StdlibSmoke test fails.

I tried to fix it with

if (enable_perfetto_etm_importer) {
  sources += [ "etm.sql" ]
}

But the conditional is ignored by the generated Android.bp and BUILD files, so this would break non-GN builds.

[LalitMaganti]

But the conditional is ignored by the generated Android.bp and BUILD files, so this would break non-GN builds.

This won't break non-GN builds. It's WAI that those builds don't have etm code.