In 1bfd54e5 (testhelper: Make injected random feature flags more flaky,
2022-05-18), we tried to avoid feature flag values being the same in
different contexts as it's likely they will run in the same
millisecond.

Unfortunately this was reverted likely accidentaly in a1d13efd
(featureflag: Remove Git v2.41 flag, 2023-06-27).

And then many people used the original buggy way to make feature flag
values random.

Let's reapply 1bfd54e5 along with a big code comment to hopefully avoid
the same mistakes.