Add more automated tests around GC.

[ruseinov]

To spot regressions it would be nice to add more tests around GC.

It's inherently slow and unreliable, the best thing I could think of is having a mainnet node run with latest main and good monitoring/log aggregation to spot those. A node would need to run at least 3 days to yield reliable results.

Another solution would be to do what @lemmih did in his branch, running the GC super often and ignoring the possibility of a fork, but that'd need a special workaround.

I don't think it's very high prio, but let's keep it in the backlog.