parse_line($string);
This test echoes a description before running the ip_defrag binary for each case, e.g.:
# ipv4 defrag
# seed = 1791903594
# PASS
A failure would abort the binary without printing 'PASS', so only the success case is recognized. The subtest name from the last 'ipv4|ipv6 ...' line is kept and used once the bare 'PASS' line shows up. Lines that do not match this format are returned unchanged.