test-pattern: substitute common compression list The compression list as to be matched with a glob because zstd might not be part of the option. By using a substitution for these, we won't have to re-glob them over and over.

# common patterns in test at can safely be replaced
from __future__ import absolute_import

substitutions = [
    # list of possible compressions
    (br'zstd,zlib,none,bzip2',
     br'$USUAL_COMPRESSIONS$'
    ),
]