tests/servers: generate temp names in /tmp for unix domain sockets

... instead of putting them in the regular pid directories because
systems generally have strict length requirements for the path name to
be shorter than 107 bytes and we easily hit that boundary otherwise.

The new concept generates two random names: one for the socks daemon and
one for http.

Reported-by: Andy Fiddaman
Fixes #11152
Closes #11166