Make `/per-rule` usable ad-hoc, for quick unit test runs

[comps]

Currently, the new /per-rule

• always reinstalls a clean VM before running the tests,
• always rsyncs everything into it,
• etc.

and is not really optimized for a use case of a CaC/content developer incrementally making changes to unit tests & quickly testing them via the /from-env test variant.

To improve this, a few things can be changed:

1. Implement CONTEST_REBUILD_CONTENT, a Contest-wide override for util.build_content() that would behave as if the function was called with rebuild=True. This is useful when a developer incrementally rsyncs small changes to a CONTEST_CONTENT location on the remote machine, without clearing the build/ directory. Forcing a rebuild incorporates the small source file changes.

2. Implement some VM-snapshot-removal function in lib.virt that would remove the external snapshot and clear any other snapshot-ish files in /var/lib/libvirt/images, and have the /per-rule test do something like:

   if not prepared for snapshot (never installed):
       g.install(...)
   else:
       g.unprepare_snapshot()
   

   effectively resulting in a regular bootable VM (to be set up via with g.booted(): like now) that the test can rsync files to. This should take only a few moments and shouldn't matter if done repeatedly. The rsync should be with --delete to cleanly update any existing content inside the (un-prepared-snapshot) VM, overriding it.

   Maybe do the un-prepare only on some variable, like CONTEST_PER_RULE_REFRESH_VM and rely on repeated test executions just re-using the same snapshot, which would be a good default for >5 slices running in ATEX, reusing remotes.

[matusmarhefka]

Note: Updates from #460 are relevant for this issue (in point 2, we no longer need to cleanup snapshot from the previous test execution).