memquota: Fix account lifetime bugs, and arrange to test mq in shadow

In https://gitlab.torproject.org/Diziet/arti/-/jobs/721773, we can see what happens when I revert one of these fixes. That demonstrates that the test arrangements are working.

However, for reasons that I don't understand, none of the logs from that job contain memquota error messages. I downloaded the artifacts zipfile and grep -R shows only the new info message about the memquota system being enabled.

Since the logs contain much information at log level TRACE, I think that the errors must be being swallowed rather than logged, somewhere in arti. Possibly, they're occuring during child account creation. I have chosen not to decorate every fallible memquota call with logging.

Thanks to @jnewsome for pointers re shadow tests. I doubt what I've done to them will be controversial, but let's ping Jim anyway.

Part of #351