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/combine_logs.py

@@ -81,7 +81,7 @@ def read_logs(tmp_dir):
     chain = glob.glob("{}/node0/*/debug.log".format(tmp_dir))
     if chain:
         chain = chain[0]  # pick the first one if more than one chain was found (should never happen)
-        chain = re.search('node0/(.+?)/debug\.log$', chain).group(1)  # extract the chain name
+        chain = re.search(r'node0/(.+?)/debug\.log$', chain).group(1)  # extract the chain name
     else:
         chain = 'regtest'  # fallback to regtest (should only happen when none exists)