ipatests: fix backup and restore
The tests for backup_and_restore check that the ipa-backup command
compresses the tar file AFTER restarting IPA services by reading the
output and looking for a pattern with "gzip" before "Starting IPA service."
As the tar file name is randomly created, it sometimes happen that the
name contains gzip and in this case the test wrongly assumes that
the gzip cmd was called.
The fix makes a stricter comparison, looking for /bin/gzip.
Fixes: https://pagure.io/freeipa/issue/8170