Add ThreadNet tests for the ledger on-disk snapshot policy

The QSM tests for ChainDB and/or LedgerDB do not currently exercise the on-disk snapshot policy: the command sequence generator instead arbitrarily chooses when to take snapshots/run the BG tasks.

This Issue is to test that the nodes write snapshots when they're expected to. I think the ThreadNet tests are a reasonable place to do that (though I may be biased by my experience there).

I'm marking this Issue as low priority, because I anticipate that the QA/Benchmark/etc teams will notice if something important goes wrong with the policy.