[3.12] gh-110756: Sync regrtest with main branch (#110779)

gh-110756: Sync regrtest with main branch

* Remove runtest.py and runtest_mp.py of Lib/test/libregrtest/.
* Backport support._parse_memlimit().

Lib/test/test_support.py

@@ -764,7 +764,45 @@ class TestSupport(unittest.TestCase):
             else:
                 self.fail("RecursionError was not raised")
 
-        #self.assertEqual(available, 2)
+    def test_parse_memlimit(self):
+        parse = support._parse_memlimit
+        KiB = 1024
+        MiB = KiB * 1024
+        GiB = MiB * 1024
+        TiB = GiB * 1024
+        self.assertEqual(parse('0k'), 0)
+        self.assertEqual(parse('3k'), 3 * KiB)
+        self.assertEqual(parse('2.4m'), int(2.4 * MiB))
+        self.assertEqual(parse('4g'), int(4 * GiB))
+        self.assertEqual(parse('1t'), TiB)
+
+        for limit in ('', '3', '3.5.10k', '10x'):
+            with self.subTest(limit=limit):
+                with self.assertRaises(ValueError):
+                    parse(limit)
+
+    def test_set_memlimit(self):
+        _4GiB = 4 * 1024 ** 3
+        TiB = 1024 ** 4
+        old_max_memuse = support.max_memuse
+        old_real_max_memuse = support.real_max_memuse
+        try:
+            if sys.maxsize > 2**32:
+                support.set_memlimit('4g')
+                self.assertEqual(support.max_memuse, _4GiB)
+                self.assertEqual(support.real_max_memuse, _4GiB)
+
+                big = 2**100 // TiB
+                support.set_memlimit(f'{big}t')
+                self.assertEqual(support.max_memuse, sys.maxsize)
+                self.assertEqual(support.real_max_memuse, big * TiB)
+            else:
+                support.set_memlimit('4g')
+                self.assertEqual(support.max_memuse, sys.maxsize)
+                self.assertEqual(support.real_max_memuse, _4GiB)
+        finally:
+            support.max_memuse = old_max_memuse
+            support.real_max_memuse = old_real_max_memuse
 
     def test_copy_python_src_ignore(self):
         # Get source directory
@@ -813,7 +851,6 @@ class TestSupport(unittest.TestCase):
     # EnvironmentVarGuard
     # transient_internet
     # run_with_locale
-    # set_memlimit
     # bigmemtest
     # precisionbigmemtest
     # bigaddrspacetest