test: specify explicit non-numeric hosts in tests

All-numeric domains are now considered invalid, since they introduce confusion: one can assume
that an all-numeric domain is a port, and the server should bind to `localhost` with that port.

The specific change to domain parsing happens in a future commit. This commit fixes the tests in
preparation for it.

Note that all-numeric top-level domains are considered invalid according to RFC 952 and RFC 1123.
In fact, the top-level domain label must be alphabetic, but we do not place that restriction, since
local domains could have arbitrary format.