Fix coredump_phnum_test when kern.compress_user_cores != 0

If kern.compress_user_cores is non-zero, decompress the core file.

Use sysctl -f to restore previous values.

Don't bother restoring ulimit -c, since that's a per-process value.

Check more commands with atf_check.

Reviewed by: olivier ngie
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D21820