bpo-33746: Fix test_unittest.testRegisterResult() in verbose mode (GH-7799)

Only make sure that the result is in unittest.signals._results, don't
check the full content of unittest.signals._results.

support._run_suite() uses TextTestRunner in verbose mode, but
TextTestRunner.run() calls registerResult(result) which made the test
fail with "odd object in result set".

Call also removeResult() to restore unittest.signals._results to
avoid test side effect.