Add test for dead-end backends

Add test for dead-end backends

The code path for launching a dead-end backend because we're out of
slots was not covered by any tests, so add one. (Some tests did hit
the case of launching a dead-end backend because the server is still
starting up, though, so the gap in our test coverage wasn't as big as
it sounds.)

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/a102f15f-eac4-4ff2-af02-f9ff209ec66f@iki.fi