burn_in_tests.py doesn't always exclude tagged tests when generating outfile

When burn_in_tests.py generates the list of executors and tests it will run, it doesn't always take tags properly into account. This can cause tests to be run in an inappropriate executor environment, which leads to spurious failures in CI.

I'm not exactly sure where the error is, but I assume it's somewhere in resmokelib.parser.create_test_membership_map().