This changes the test runner to automatically treat cases in which
both a flag and its negation are present as a flag contradiction.
Example: "--foo --no-foo".

With this change it's no longer necessary to explicitly specify these
trivial contraditions in variants.py.

Note: since negations are created through simple string operations,
bogus constructions are possible ("--nobodys-perfect" ->
"--bodys-perfect"). We accept these as unlikely-to-cause-problems.

Bug: v8:10577
Change-Id: Ic52a92ed1e884b495ee4136f6e2f3257cca243c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2904218Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74773}