Add parallel front-end test suite to compiletest

[SparrowLii]

Parallel front end need some tests in CI, to prevent certain changes from making it unavailable. (e.g. #113730)

### Tasks
- [x] Modify compiler test harness to support repeated testing
- [x] Testing with many threads
- [x] use parallel front end in UI tests
- [ ] make the parallel UI tests blocking on CI

[petrochenkov]

A way to run UI test suite in parallel mode was added in #153801, and #157705 adds it to CI as a non-blocking job.
The remaining part is to make it blocking on CI.

The rust-perf part has a separate tracking issue - rust-lang/rustc-perf#2068, I'll remove it from the list here.