Only actually needed servers should be listed and none is then implied
if no servers are listed.
Outputs a warning if "none" is still set as a server.
Closes#18466
`runtests.pl` defaults to `-` if a command is not set, since
c43ad0f972.
Also:
- drop a stray `</file>`.
- replace a `nothing` with a guaranteed invalid hostname.
Ref: 4334033b43
- replace unused URLs with `-`.
Closes#18319
