This isn't reliable. There's no guarantee on the order of filesystems returned by mount. If you want to check the root partition, you should try something like mount | awk '$3 == "/"'

Also, you seem to be using this as a gate for whether to run tests. In that case, it's inappropriate to ues atf_check, because that will fail the test if ACLs aren't enabled. Instead, you should skip the test if ACLs aren't enabled.

Finally, Kyua doesn't use / for temporary files. Instead, it uses $TMPDIR. But you shouldn't rely on that. Instead, you should check the filesystem where you happen to be executing. Try something like this:

fs=`df . | awk '$NF ~ "/" {print $1}'`
if mount | awk -v fs=$fs '$1 == fs' | grep -q acls; then
    # ok
else
   atf_skip "ACLs not enabled"
fi

This will only work for POSIX.1e ACLs. FreeBSD supports two incompatible ACL formats: POSIX.1e and NFSv4. I think the best thing to do would be to write two sets of tests: one for each ACL format. On any one run, one set would be skipped, depending on what filesystem Kyua is using. Or, you could write tests just for POSIX.1e ACLs, because that's what FreeBSD's CI system uses. In that case, you should modify check_acl to exclude filesystems with NFSv4 ACLs.