test: Test the same things with unix: that we do with tcp:

test: Test the same things with unix: that we do with tcp:

Minimal autobuilder environments don't always have working TCP,
so we may need to skip TCP tests. Make sure we test the equivalent
code paths via Unix sockets in those environments.

One notable exception is test/fdpass.c, which uses TCP as a transport
that is known not to be able to carry Unix fds; this needs to continue
to use TCP.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=106812
(cherry picked from commit cb7dd5bfccb2882d0e9951c41040ff6a97bb827d)