tests: Add test to verify HiFi save/restore

Adds a test to verify the saving and restoration of the HiFi
AudioEngine registers. Twice as many threads as there are CPUs
are created and each thread sets its HiFi registers with a
unique set of values. If after a context switch, the HiFi AE
registers contain any unexpected data, we then know that the
registers were either not saved or restored properly.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>