tests: input/api: limit to 1 CPU for thread mode test

There is a check to see if it is no longer able to push more
messages into a full queue. When these is another CPU consuming
messages, the queue would not be full at that point. A new message
can be pushed into the queue and thus failing the "full queue"
check. So limit this to 1 CPU only so this check's assumption
can be fulfilled.

Fixes #79319

Signed-off-by: Daniel Leung <daniel.leung@intel.com>