Fix --blacklist when --jobs > 1 in run_tests.py. The options parsing logic replaces the options.blacklist initial value, a list of filenames, with a dict of test names to filenames. When runchildren rebuilds the command line to launch child processes the dict is flattened and provided as a malformed argument when a filename is expected. Fix is to remove the blacklist option from child invocations since the filtering is handled in the parent anyway.