tests: cmsis_rtos_v2: fix thread checks

It's really not possible to design a test where we can
enforce expectations on real vs. expected stack size:

- Some platforms may increase stack size over what is expected
  due to rounding up the stack buffer area to the next power
  of two.

- Some configuration options like CONFIG_STACK_RANDOM
  carve out space in the stack buffer, resulting in a stack
  size less than what is expected.

Best we can do is just assert that the amount of space
available should be less than the total size reported.

Fixes: #14640

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>