Consider timed tests

[MattWindsor91]

This is a holding issue for the possibility of considering time in rtcg-accepted and generated tests. Such an addition would likely require a large overhaul of how rtcg internally considers tests (i.e. they are no longer straightforward chains of outcome-in/out-outcome-in/out-...), and is also dependent on any timed testing work becoming sufficiently impactful to necessitate the workload (and, of course, time permitting!).