Add two tests for the multiplex rotation engine.

pmc_mux_rotation_test checks that rotation is fair, that every member of
an oversubscribed group gets time on the hardware, that a pinned group
keeps its rows, and that the rotation thread stops when no group needs
it any more.

pmc_mux_drain_test reproduces the drain wedge at switch-off. It runs
pinned real-time worker threads that starve a passive drain, so it needs
-lpthread.

Sponsored by: AMD

Signed-off-by: Andre Silva <andasilv@amd.com>