tests/kernel/fpu_sharing: Run test with MP_NUM_CPUS=1

This test uses k_yield() to "sync" between threads, so it's implicitly
supposed to run on a single CPU. Make it explicit, to avoid issues on
platforms with more cores.

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

FIXKFLOATDISABLE