soc/xtensa/intel_adsp/cavs: Fix CPU sporadically not idling

Sometimes, on CAVS 25, the CPU may fail to idle. This was due a failure
during test initialisation - the test is supposed to run from CPU0, and
halt and restart the other ones. However, code to pin thread to CPU was
incorrect - a running thread can't be pinned to a CPU (the
k_thread_cpu_mask* calls were returning error).

This patch solves this by creating a thread that is pinned to CPU0
before it is run.

While at it, add some more checks for returned values.

Fixes: #49790

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>