XFAIL TestLocalPath.test_make_numbered_dir_multiprocess_safe (#11611)

The tested py.path.local.make_numbered_dir function is *not*
multiprocess safe, because is uses os.listdir which itself is not.

The os.listdir documentation explicitly states that:

> If a file is removed from or added to the directory during the call
> of this function, whether a name for that file be included is unspecified.

This can lead to a race when:

 1. process A attempts to create directory N
 2. the creation fails, as another process already created it in the meantime
 3. process A calls listdir to determine a more recent maxnum
 4. processes B+ repeatedly create newer directories and they delete directory N
 5. process A doesn't have directory N or any newer directory in listdir result
 6. process A attempts to create directory N again and raises

For details, see https://github.com/pytest-dev/pytest/issues/11603#issuecomment-1805708144
and bellow.

Additionally, the test itself has a race in batch_make_numbered_dirs.
When this functions attempts to write to repro-N/foo,
repro-N may have already been removed by another process.

For details, see https://github.com/pytest-dev/pytest/issues/11603#issuecomment-1804714313
and bellow.

---

The tested py.path.local.make_numbered_dir function is not used in pytest.
There is a different implementation in _pytest.pathlib.

Closes #11603

testing/_py/test_local.py

@@ -868,6 +868,9 @@ class TestLocalPath(CommonFSTests):
             py_path.strpath, str_path
         )
 
+    @pytest.mark.xfail(
+        reason="#11603", raises=(error.EEXIST, error.ENOENT), strict=False
+    )
     def test_make_numbered_dir_multiprocess_safe(self, tmpdir):
         # https://github.com/pytest-dev/py/issues/30
         with multiprocessing.Pool() as pool: