tests: Use different connections for read and write tests
tests: Use different connections for read and write tests

The connection tests so far only use one connection at a time, but this
prepares for tests that use a connection on both sides of the socket.

sir-murray/wayland,tulcod/wayland,mchalupa/wayland,tulcod/wayland,abooij/wayland,mchalupa/wayland,bitfactor/wayland,tulcod/wayland,eyolfson/wayland,mchalupa/wayland,eyolfson/wayland,sir-murray/wayland,eyolfson/wayland,abooij/wayland,bitfactor/wayland,sir-murray/wayland,bitfactor/wayland,abooij/wayland