pythongh-108388: Convert test_concurrent_futures to package

* Split test_concurrent_futures.py into multiple files
  in a new Lib/test/test_concurrent_futures/ package.
* Add remote_globals to create_executor_tests()

Author: vstinner

Lib/test/libregrtest/runtest.py

@@ -132,6 +132,7 @@ def __str__(self) -> str:
 
 SPLITTESTDIRS = {
     "test_asyncio",
+    "test_concurrent_futures",
     "test_multiprocessing_fork",
     "test_multiprocessing_forkserver",
     "test_multiprocessing_spawn",