fdpass test: assert that TCP connections don't say they can pass fds

fdpass test: assert that TCP connections don't say they can pass fds

As well as slightly increasing our test coverage, this ensures
that at least one test-case in this test is not skipped, working
around CMake's lack of support for the Automake-style exit code
produced by GTest executables (they exit 77 if everything was skipped).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=92887
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>