qa: Add TestNode::assert_debug_log #14024

Passing in an escaped string means that the printed assert when this fails looks a bit weird, but no big deal:

    raise AssertionError(self._node_msg(msg))
AssertionError: [node 0] Expected message "8d7190866370e089916899a483a8df5ce7b495b04ffefc841ecd0d5779e9a0ca\ asdf\ from\ peer\=0\ was\ not\ accepted\:\ mandatory\-script\-verify\-flag\-failed\ \(Invalid\ OP_IF\ construction\)\ \(code\ 16\)" does not partially match log:

Printing the log excerpt out in the assert is a bit confusing for anything that's parsing the test_framework log (since the timestamps at the start of the line will be interpreted as new log lines. Can I convince you to make the following change:

@@ -242,9 +242,10 @@ class TestNode():
             with open(debug_log, encoding='utf-8') as dl:
                 dl.seek(prev_size)
                 log = dl.read()
+                print_log = " - " + "\n - ".join(log.splitlines())
             for expected_msg in expected_msgs:
                 if re.search(expected_msg, log, flags=re.MULTILINE) is None:
-                    self._raise_assertion_error('Expected message "{}" does not partially match log:\n"{}"\n'.format(expected_msg, log))
+                    self._raise_assertion_error('Expected message "{}" does not partially match log:\n\n{}\n\n'.format(expected_msg, print_log))

so that the printed log lines are prefixed with -.

I'd also be tempted to add a hint like:

        self.log.error("Expected Log not raised. Check that bitcoind log format output has not changed!")

Changes to logging that cause tests to fail can be really annoying, so adding a hint is a friendly thing to do here.

Might be more efficient to move this line before the _raise_assertion_error call in case the variable is unused.

I think we don't optimize for performance in the test framework, so this should be fine.

I was surprised to see that this code actually works without adding RPCs before and after the yield to flush the log. But apparently there is a setbuf call in the logging code which disables buffering. Might be worth noting in a comment.