check-libcxx: Introduce config.test_retry_attempts for std/thread.

[chapuni]

Summary

I thought we could mark entire tests in std/thread as flaky.

libcxx/trunk/test/std/thread/lit.local.cfg

+config.test_retry_attempts = 3

libcxx/trunk/utils/libcxx/test/format.py

def _evaluate_pass_test(self, test, tmpBase, lit_config,
             # TODO: Only list actually needed files in file_deps.
             # Right now we just mark all of the .dat files in the same
             # directory as dependencies, but it's likely less than that. We
             # should add a `// FILE-DEP: foo.dat` to each test to track this.
             data_files = [os.path.join(local_cwd, f)
                           for f in os.listdir(local_cwd) if f.endswith('.dat')]
             is_flaky = self._get_parser('FLAKY_TEST.', parsers).getValue()
             max_retry = 3 if is_flaky else 1
+            local_flaky = getattr(test.config, "test_retry_attempts", 0)
+            if local_flaky > 0:
+                max_retry = local_flaky
             for retry_count in range(max_retry):
                 cmd, out, err, rc = self.executor.run(exec_path, [exec_path],
                                                       local_cwd, data_files,
                                                       env)
                 if rc == 0:
                     res = lit.Test.PASS if retry_count == 0 else lit.Test.FLAKYPASS
                     return res, ''
                 elif rc != 0 and retry_count + 1 == max_retry: