test/contrib: Fix invalid escapes in regex strings

Flagged by flake8 v3.6.0, as W605, plus a few others identified
incidentally, e.g. 59ffecf66cf4d08c4b431e457b083878d66a3fd6.

Note that r"\n" matches to "\n" under re.match/search.

test/functional/feature_logging.py

@@ -36,7 +36,7 @@ class LoggingTest(BitcoinTestFramework):
         invdir = self.relative_log_path("foo")
         invalidname = os.path.join("foo", "foo.log")
         self.stop_node(0)
-        exp_stderr = "Error: Could not open debug log file \S+$"
+        exp_stderr = r"Error: Could not open debug log file \S+$"
         self.nodes[0].assert_start_raises_init_error(["-debuglogfile=%s" % (invalidname)], exp_stderr, match=ErrorMatch.FULL_REGEX)
         assert not os.path.isfile(os.path.join(invdir, "foo.log"))