tests/kernel/thread_stack: Increase heap size

This test was written to use a TINY system heap (64 bytes) from which
it has to allocate on behalf of a userspace process.  The change in
convention from mem_pool (where the byte count now includes metadata
overhead) means it runs out of space.  Bump to 192 bytes.  Still tiny.

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