Issue 7127: regrtest -j fails when tests write to stderr

If a test writes to stderr, then the -j (multiprocessing) support in
regrtest fails to correctly extract the JSON data from the test output,
resulting in a JSON traceback followed by a failure of the worker
thread.  Antoine has suggested tagging the JSON result string with a
prefix and looking for that prefix, instead of the current method (which
assumes that the JSON result string is at the end of the output from the
test).