The in-memory identity creation test was failing intermittently because
the server only creates the in-memory identity on the first tick, and
under load the server sometimes does not tick for a while, leading to a
race with the "servald id self" command.
The test case now waits for the server to emit a tick before running the
"id self" command. This necessitated a new DEBUG() statement and a new
debug flag 'overlaytick'.
Also disabled Rhizome for all the keyring tests, as unnecessary Rhizome
DB creation was slowing down the fixtures.
43bd0bd33f