While in itself this is not a slow test, on a loaded system the
connection failures can degrade the run time dramatically, like 20
times slower.

To save some time here and there, some client sessions were folded
together where it made sense. Also no two clients share the same name
to make it easier to spot the offending one in a test log.

More importantly, the holddown test at the end which concerns the very
first VCL is now put at the beginning where it belongs, and also where
tweaking backend_remote_error_holddown will actually have the desired
effect.