com/info: Use "dplint info" in usage when called as such

Signed-off-by: Niels Thykier <niels@thykier.net>
lintian: Let --color default to "auto" instead of "never"

Signed-off-by: Niels Thykier <niels@thykier.net>
t/runtests: Make $RUNDIR absolute since builders rely on it

Some of the builders rely on $RUNDIR being absolute, so resolve it to
avoid issues for people using t/runtests directly.

Signed-off-by: Niels Thykier <niels@thykier.net>
t/runtests: Update usage information

Signed-off-by: Niels Thykier <niels@thykier.net>
t/runtests: Remove -t <tag> (replaced by "tag:<tag>")

Signed-off-by: Niels Thykier <niels@thykier.net>
d/changelog: Add closes for #699083

With the artifact caching in t/runtests, subsequent runs can now be
done vastly faster.

Signed-off-by: Niels Thykier <niels@thykier.net>