tests/kernel/mem_pool: Fix timeout units

This code was clearly written to assume that the timeout argument to
k_mem_pool_alloc() was in ticks and not ms.  Adjust to what appears to
have been the intent.  It was working as intended (i.e waiting one or
1/10th of a second) only on systems where the default tick rate was
100 Hz.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>