Attempt to run parallel frontend CI

[heinwol]

WIP

Supersedes #157705.

The issue with the original change was arguments --parallel-frontend-threads=4 --iteration-count=2 being compiletest-only. They were being passed to other parts of the test harness (e.g. libtest) as is. These parts, however, have no clue of the arguments, hence the errors.

I've spent a lot of time on this and haven't found any reasonable way to fix this behavior. We could make bootstrap aware of these specific two arguments and have an additional internal logic for handling this. It's big of a hack, i reckon. The best decision i arrived at is to split testing into two parts: one for compiletest only and another for everything else. The issue is (AFAIK) we can't tell bootstrap (or x.py, at least) to "test the default stuff, but only for compiletest". When used like x test tests/ it runs all the tests in this directory, including non-default ones, and crashes as it can't find nodejs for doctests. --skip compiler/ --skip library/ --skip src/tools/ --skip tests/incremental ... is still not exhaustive list of exclusions.

I went on with a whitelist instead of a blacklist. But we, again, can't tell what tests are "default". There's a mechanism in bootstrap::core::builder::Builder for showing "dry-run" test suites, but it's not available from the cli. For now, my whitelist is quite small and i have no idea what should it be like.

Also, i added ENV RUST_TEST_THREADS=1 just because it was introduced by @ zetanumbers; i'm not sure it's necessary. It ran ok in the previous iteration.

Finally, i'm not sure it works for now, i'm still spending hours blindly guessing the maxumum working set of tests. I've decided to make the pr now just to have something done and seek advice.

r? @petrochenkov