tests: ipc: refactor/split test_ipc_dispatch part into client_dispatch
tests: ipc: refactor/split test_ipc_dispatch part into client_dispatch

This way, this core part can be easily reused where needed.
Note that "ready_signaller" similarity with run_ipc_server is not
accidental, following commit will justify it.

Signed-off-by: Jan Pokorný <f06ec55ecfb6704a26166905f43ade3ead1f75d3@redhat.com>

kgaillot/libqb,ClusterLabs/libqb,ClusterLabs/libqb,kgaillot/libqb,ClusterLabs/libqb,kgaillot/libqb