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/test_framework/test_framework.py

@@ -10,6 +10,7 @@ import argparse
 from datetime import datetime, timezone
 import logging
 import os
+from pathlib import Path
 import platform
 import pdb
 import random
@@ -331,7 +332,7 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
             h.flush()
             rpc_logger.removeHandler(h)
         if cleanup_tree_on_exit:
-            shutil.rmtree(self.options.tmpdir)
+            self.cleanup_folder(self.options.tmpdir)
 
         self.nodes.clear()
         return exit_code
@@ -1031,3 +1032,9 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
             return result
         except ImportError:
             self.log.warning("sqlite3 module not available, skipping tests that inspect the database")
+
+    def cleanup_folder(self, _path):
+        path = Path(_path)
+        if not path.is_relative_to(self.options.tmpdir):
+            raise AssertionError(f"Trying to delete #{path} outside of #{self.options.tmpdir}")
+        shutil.rmtree(path)