tests: Use skip_without_ostree_feature to detect libarchive, composefs

tests: Use skip_without_ostree_feature to detect libarchive, composefs

This avoids false negatives from `ostree --version | grep -q ...`
exiting with failure under `set -o pipefail` because `grep -q` can exit
as soon as it sees the desired string, leaving `ostree --version` to be
terminated by `SIGPIPE` next time it writes to stdout.

Signed-off-by: Simon McVittie <smcv@collabora.com>