include test-results & "trash" directory contents in build output
    
    When a build fails, the contents of t/test-results and the trash
    directories can be quite useful for debugging.  This is particularly
    true when the failures occur only in Koji, where we can't get a shell
    and poke around.
    
    Create a compressed tarball and encode it with base64 to allow it to be
    output along with the normal build output.  Include instruction on how
    to extract the base64-encoded content from the build log inline.
    
    The tar archive is compressed with zstd which provides a good balance of
    speed and size.  The compression level of 17 was chosen after a number
    of tests against real test failures, as opposed to entirely random
    selection. ;)