tests: kernel: fp_sharing: increase robustness of float_disable test

This commit make the float_disable test suite more robust
for fast CPUs, by replacing k_sleep(1) with k_yield(), as
the mechanism to trigger thread swap-out during the test
execution. In the wake of using k_yield(), the test, now,
fixes the priorities of all testing threads to 0, making
the test behavior more deterministic with respect to
thread scheduling. The patch doesn't change the functional
behavior of the test.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>