adapt concat_compress unit tests to gzip block sequence

adapt concat_compress unit tests to gzip block sequence

The unit tests assume that compression of three files requires
three distinct Gzip blocks. The first one of these is empty and
serves no purpose, differing from the others by containing the
more or less redundant archive name. This is no longer the case
after the revision of the header code: the first block will still
have the archive name in the metadata but also contain the first
file.

Thus, adapt the unit tests to no longer check for and then ignore
the empty initial gzip block.