gh-152548: Convert test_audit to the test.support.isolated() decorator

Each audit test needs a fresh interpreter because sys.addaudithook()
cannot be undone.  @support.isolated() provides one, so the tests become
ordinary TestCase methods using self.assert* in the subprocess, dropping
the audit-tests.py driver script and the code that parsed events out of
its stdout.

test_excepthook stays a subprocess test: the "sys.excepthook" audit
event only fires for an exception reaching the interpreter's top level,
which an @isolated() body (run under unittest) never does.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka