bpo-37069: regrtest uses sys.unraisablehook (GH-13759)

regrtest now uses sys.unraisablehook() to mark a test as "environment
altered" (ENV_CHANGED) if it emits an "unraisable exception".
Moreover, regrtest logs a warning in this case.

Use "python3 -m test --fail-env-changed" to catch unraisable
exceptions in tests.