tests: kernel: thread: increase heap mem pool size

Increase the heap memory pool size in the
prj_armv8m_mpu_stack_guard.conf, to match
the value in the default configuration in
proj.conf (and fix an out-of memory issue
when allocating a kernel object).

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