Fix test output when gzip is run with -tlv

When run with test, verbose and list we need to parse the file otherwise
the test output is "NOT OK" even for the file is valid.

Reviewed by: kevans, allanjude, imp
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D34046

(cherry picked from commit 21c966a6b9f2e9110b8dc2a822ad2b4583f4f18b)