Python: Add test with prefix

Author: yoff

python/ql/test/query-tests/Security/CWE-730-ReDoS/unittests.py

@@ -10,3 +10,4 @@
 re.compile(r'(?i)(?:.|\n)*b') # No ReDoS.
 re.compile(r'(?s)(?:.|\n)*b') # Has ReDoS.
 re.compile(r'(?is)(?:.|\n)*b') # Has ReDoS.
+re.compile(r'(?is)X(?:.|\n)*Y') # Has ReDoS.