tests/lib/ringbuffer: Run with one CPU

The stress ('scuse me, "zstress") cases here are all written to
exercise reader/writer threads at different priority combinations.
That's defeated if the threads are allowed to run on different CPUs
(because being "low" priority doesn't matter if you have a spare CPU
to run on).

There is also extensive use of stack buffers to pass data through the
ring buffer zero copy implementation, which runs afoul of the
KERNEL_COHERENCE rules on intel_adsp platforms (where stack memory is
incoherent between CPUs and can't be shared like that).

Fix both issues by just setting CONFIG_MP_NUM_CPUS=1

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>