Flip --experimental_use_validation_aspect

[fmeum]

RELNOTES: Validation actions now run concurrently with tests.

[fmeum]

@lberki I learned that this flag is essentially always enabled at Google and think that it would be the more useful default for Bazel as well (with #23589 fixed).

[fmeum]

Okay, there are many more BES tests checking exact counts than I imagined. I can fix them, but this leads to the question whether we would want to exclude the ValidateTarget aspect from the BES or represent it differently. @lberki What do you think? How are you handling this at Google?

[lberki]

I'm afraid I must disappoint you here -- the way our internal integration test suite works is that the flags in the repository-level bazelrc do not track their production values closely. Every once in a while, we get the idea to synchronize them, but there are good reasons for them to diverge so it never comes to anything :(

It's actually even worse than that because we only use --experimental_use_validation_aspect in most of our builds: some systems built around Blaze got confused by the very extra BES messages these tests complain about and we haven't get gotten around fixing them...

My ideal course of action would be to flip the flag and update the tests nevertheless, thereby moving the needle towards this flag being on. I'd totally tell how how many tests we have internally that break, but I'm afraid I'm somewhat oversubscribed today so I can't...