Rollup merge of #146155 - Shourya742:2025-09-03-make-test-run-parallel, r=Kobzol

Make bootstrap self test parallel

This PR refactors the bootstrap self test:

1. Replaced the hardcoded --test-threads=1 with std::thread::available_parallelism(), allowing tests to run with the maximum concurrency supported by the system.
2. Replaced custom hacks around OUT_DIR with TempDir and introduced TestCtx to simplify configuration setup.

r? `@Kobzol`

Author: Zalathar