[3.12] gh-108388: Convert test_concurrent_futures to package (#108401) (#108443)

gh-108388: Convert test_concurrent_futures to package (#108401)

Convert test_concurrent_futures to a package of sub-tests.

(cherry picked from commit aa6f787)

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",