gh-138122: Make sampling profiler integration tests more resilient (#142382)

The tests were flaky on slow machines because subprocesses could finish
before enough samples were collected. This adds synchronization similar
to test_external_inspection: test scripts now signal when they start
working, and the profiler waits for this signal before sampling.

Test scripts now run in infinite loops until killed rather than for
fixed iterations, ensuring the profiler always has active work to
sample regardless of machine speed.