Parallel frontend testing on CI

NOTE: this job is optional and allowed to fail.

If you see any test failures in tests/ui from the CI job x86_64-gnu-parallel-frontend, please add //@ ignore-parallel-frontend triage to the failing test, even if your PR is otherwise entirely unrelated to parallel compiler or its testing. In some time people from the parallel rustc working group will triage the failing test, make a tracking issue for it, and try to debug the problem.

For more context, see:

• MCP: Stabilization strategy for rustc parallel frontend
• Tracking issue: https://github.com/rust-lang/rust/issues/118698