bpo-30028: Add comments, move the test to test_support.py

Added an explanatory comment and moved the test to test_support.py.
I also modified the test to be in line with the other tests of temp_cwd.

Lib/test/support/__init__.py

@@ -965,6 +965,8 @@ def temp_dir(path=None, quiet=False):
     try:
         yield path
     finally:
+        # In case the process forks, let only the parent remove the
+        # directory. The child has a diffent process id. (bpo-30028)
         if dir_created and pid == os.getpid():
             rmtree(path)
 

Lib/test/test_regrtest.py

@@ -821,19 +821,5 @@ def test_crashed(self):
                                   randomize=True)
 
 
-class TempCwdTestCase(unittest.TestCase):
-    @unittest.skipUnless(hasattr(os, "fork"), "test requires os.fork")
-    def test_forked_child(self):
-        with support.temp_cwd() as cwd:
-            pid = os.fork()
-            if pid != 0:
-                # parent
-                os.waitpid(pid, 0)
-                self.assertTrue(os.path.isdir(cwd), "directory was removed "+cwd)
-        if pid == 0:
-            # terminate the child in order to not confuse the test runner
-            os._exit(0)
-
-
 if __name__ == '__main__':
     unittest.main()

Lib/test/test_support.py

@@ -161,6 +161,20 @@ def test_temp_dir__existing_dir__quiet_true(self):
                                         f'temporary directory {path!r}: '),
                         warn)
 
+    @unittest.skipUnless(hasattr(os, "fork"), "test requires os.fork")
+    def test_temp_dir__forked_child(self):
+        """Test that a forked child process does not remove the directory."""
+        with support.temp_cwd() as temp_path:
+            pid = os.fork()
+            if pid != 0:
+                # parent process
+                os.waitpid(pid, 0)  # wait for the child to terminate
+                # make sure that temp_path is still present
+                self.assertTrue(os.path.isdir(temp_path))
+        if pid == 0:
+            # terminate the child in order to not confuse the test runner
+            os._exit(0)
+
     # Tests for change_cwd()
 
     def test_change_cwd(self):