objectstore: extract remove_tree()

Move remove_tree() into its own module in `osbuild.util.rmrf`. This way
we can use it in other modules as well, without cross-referencing
internal helpers.

test/test_objectstore.py

@@ -2,25 +2,11 @@ import os
 import shutil
 import tempfile
 import unittest
-import subprocess
 
 from pathlib import Path
 from osbuild import objectstore
 
 
-def can_set_immutable():
-    with tempfile.TemporaryDirectory(dir="/var/tmp") as tmp:
-        try:
-            os.makedirs(f"{tmp}/f")
-            # fist they give it ...
-            subprocess.run(["chattr", "+i", f"{tmp}/f"], check=True)
-            # ... then they take it away
-            subprocess.run(["chattr", "-i", f"{tmp}/f"], check=True)
-        except (subprocess.CalledProcessError, FileNotFoundError):
-            return False
-        return True
-
-
 class TestObjectStore(unittest.TestCase):
 
     @classmethod
@@ -87,18 +73,6 @@ class TestObjectStore(unittest.TestCase):
             # there should be no temporary Objects dirs anymore
             self.assertEqual(len(os.listdir(object_store.tmp)), 0)
 
-    # pylint: disable=no-self-use
-    @unittest.skipUnless(can_set_immutable(), "Need root permissions")
-    def test_cleanup_immutable(self):
-        with tempfile.TemporaryDirectory(dir="/var/tmp") as tmp:
-            with objectstore.ObjectStore(tmp) as object_store:
-                tree = object_store.new()
-                with tree.write() as path:
-                    p = Path(f"{path}/A")
-                    p.touch()
-                    subprocess.run(["chattr", "+i", f"{path}/A"],
-                                   check=True)
-
     # pylint: disable=no-self-use
     def test_duplicate(self):
         with tempfile.TemporaryDirectory(dir="/var/tmp") as tmp: