Revert "tests: do not use lazy unmount"

Revert to use umount -l in storage tests cleanup. With fixed permissions
in 4234fe51 "tests: fix cleanup after reflink tests", it shouldn't cause
issues anymore, but apparently on some systems test cleanup fails
otherwise.

Reported by @rustybird
This reverts commit b6f77ebfa1.

qubes/tests/storage_reflink.py

@@ -135,11 +135,7 @@ def rmtree_fs(directory):
     cmd('sudo', 'chattr', '-i', directory)
     cmd('sudo', 'chmod', '777', directory)
     if os.path.ismount(directory):
-        try:
-            cmd('sudo', 'umount', directory)
-        except:
-            cmd('sudo', 'fuser', '-vm', directory)
-            raise
+        cmd('sudo', 'umount', '-l', directory)
         # loop device and backing file are garbage collected automatically
     shutil.rmtree(directory)