test: functional: drop rmtree usage and add lint check

`shutil.rmtree` is dangerous because it recursively deletes. There are
not likely to be any issues with it's current uses, but it is possible
that some of the assumptions being made now won't always be true, e.g.
about what some of the variables being passed to `rmtree` represent.

For some remaining uses of rmtree that can't be avoided for now, use
`cleanup_dir` which asserts that the recursively deleted folder is a
child of the the `tmpdir` of the test run. Otherwise,
`tempfile.TemporaryDirectory` should be used which does it's own
deleting on being garbage collected, or old fashioned unlinking and
rmdir in the case of directories with known contents.

test/functional/wallet_backwards_compatibility.py

@@ -303,7 +303,8 @@ class BackwardsCompatibilityTest(BitcoinTestFramework):
                     # Copy back to master
                     wallet.unloadwallet()
                     if n == node:
-                        shutil.rmtree(node_master.wallets_path / wallet_name)
+                        (node_master.wallets_path / wallet_name / "wallet.dat").unlink()
+                        (node_master.wallets_path / wallet_name).rmdir()
                         shutil.copytree(
                             n.wallets_path / wallet_name,
                             node_master.wallets_path / wallet_name,