gh-109276: Enhance libregrtest results

[vstinner]

• Factorize code listing "bad / env changed / ..." tests.
• Add TestResults.is_all_good() method.
• Move "All 400 tests OK." to the end
• Move "Test suite interrupted by signal SIGINT." to the end.

• Issue: Enhance Python regrtest test runner (test.libregrtest) #109276

[vstinner]

Not good, 4 tests failed randomly (failed then pass) on Windows (x64):

• test_asyncio.test_waitfor: issue test_asyncio.test_waitfor: test_wait_for() failed with "AssertionError: 0.547 not less than 0.5" on GHA Windows x64 #109833
• test_concurrent_futures.test_as_completed: issue test_concurrent_futures: test_as_completed.test_future_times_out() fails on GHA Windows x86 CI #109565
• test_regrtest: issue test_regrtest: test_huntrleaks() fails randomly: leaked [9, 1, 1] references #109739
• test_threading: issue test_threading: test_default_timeout() fails randomly ands logs "Warning -- Unraisable exception" #109401

[vstinner]

Second Windows x64 job, a new test failed: issue gh-109840.

FAIL: test_waitfor_timeout (test.test_multiprocessing_spawn.test_processes.WithProcessesTestCondition.test_waitfor_timeout)