gh-109162: libregrtest: add single.py and result.py (#109243)

* Add single.py and result.py files.
* Rename runtest.py to runtests.py.
* Move run_single_test() function and its helper functions to
  single.py.
* Move remove_testfn(), abs_module_name() and normalize_test_name()
  to utils.py.
* Move setup_support() to setup.py.
* Move type hints like TestName to utils.py.
* Rename runtest.py to runtests.py.

Lib/test/libregrtest/findtests.py

@@ -0,0 +1,58 @@
+import os
+
+from test.libregrtest.utils import StrPath, TestName, TestList
+
+
+# If these test directories are encountered recurse into them and treat each
+# "test_*.py" file or each sub-directory as a separate test module. This can
+# increase parallelism.
+#
+# Beware this can't generally be done for any directory with sub-tests as the
+# __init__.py may do things which alter what tests are to be run.
+SPLITTESTDIRS: set[TestName] = {
+    "test_asyncio",
+    "test_concurrent_futures",
+    "test_multiprocessing_fork",
+    "test_multiprocessing_forkserver",
+    "test_multiprocessing_spawn",
+}
+
+
+def findtestdir(path=None):
+    return path or os.path.dirname(os.path.dirname(__file__)) or os.curdir
+
+
+def findtests(*, testdir: StrPath | None = None, exclude=(),
+              split_test_dirs: set[TestName] = SPLITTESTDIRS,
+              base_mod: str = "") -> TestList:
+    """Return a list of all applicable test modules."""
+    testdir = findtestdir(testdir)
+    tests = []
+    for name in os.listdir(testdir):
+        mod, ext = os.path.splitext(name)
+        if (not mod.startswith("test_")) or (mod in exclude):
+            continue
+        if mod in split_test_dirs:
+            subdir = os.path.join(testdir, mod)
+            mod = f"{base_mod or 'test'}.{mod}"
+            tests.extend(findtests(testdir=subdir, exclude=exclude,
+                                   split_test_dirs=split_test_dirs,
+                                   base_mod=mod))
+        elif ext in (".py", ""):
+            tests.append(f"{base_mod}.{mod}" if base_mod else mod)
+    return sorted(tests)
+
+
+def split_test_packages(tests, *, testdir: StrPath | None = None, exclude=(),
+                        split_test_dirs=SPLITTESTDIRS):
+    testdir = findtestdir(testdir)
+    splitted = []
+    for name in tests:
+        if name in split_test_dirs:
+            subdir = os.path.join(testdir, name)
+            splitted.extend(findtests(testdir=subdir, exclude=exclude,
+                                      split_test_dirs=split_test_dirs,
+                                      base_mod=name))
+        else:
+            splitted.append(name)
+    return splitted