test framework: expand expected_stderr, expected_ret_code options

Allow `expected_stderr` option passed to `wait_until_stopped` and
`is_node_stopped` helper functions to be a regex pattern instead of just a
fixed string.

Allow `expected_ret_code` be list of possible exit codes instead of a single
error code to handle the case where exit codes vary depending on OS and libc.

test/functional/test_framework/test_node.py

@@ -22,6 +22,7 @@ import collections
 import shlex
 import shutil
 import sys
+from collections.abc import Iterable
 from pathlib import Path
 
 from .authproxy import (
@@ -469,6 +470,12 @@ class TestNode():
         """Checks whether the node has stopped.
 
         Returns True if the node has stopped. False otherwise.
+
+        If the process has exited, asserts that the exit code matches
+        `expected_ret_code` (which may be a single value or an iterable of values),
+        and that stderr matches `expected_stderr` exactly or, if a regex pattern is
+        provided, contains the pattern.
+
         This method is responsible for freeing resources (self.process)."""
         if not self.running:
             return True
@@ -477,12 +484,17 @@ class TestNode():
             return False
 
         # process has stopped. Assert that it didn't return an error code.
-        assert return_code == expected_ret_code, self._node_msg(
+        if not isinstance(expected_ret_code, Iterable):
+            expected_ret_code = (expected_ret_code,)
+        assert return_code in expected_ret_code, self._node_msg(
             f"Node returned unexpected exit code ({return_code}) vs ({expected_ret_code}) when stopping")
         # Check that stderr is as expected
         self.stderr.seek(0)
         stderr = self.stderr.read().decode('utf-8').strip()
-        if stderr != expected_stderr:
+        if isinstance(expected_stderr, re.Pattern):
+            if not expected_stderr.search(stderr):
+                raise AssertionError(f"Unexpected stderr {stderr!r} does not contain {expected_stderr.pattern!r}")
+        elif stderr != expected_stderr:
             raise AssertionError("Unexpected stderr {} != {}".format(stderr, expected_stderr))
 
         self.stdout.close()