tests: preempt: increase stack size for test

Failed with:

Running test suite suite_preempt
===================================================================
starting test - test_preempt
***** Stack Check Fail! *****
Current thread ID = 0x0040019c
eax: 0x00400254, ebx: 0x00400254, ecx: 0x004002b0, edx: 0x00000001
esi: 0x004001f8, edi: 0x00401080, ebp: 0x00408024, esp: 0x00408000
eflags: 0x00000046 cs: 0x0008
call trace:
eip: 0x00002115
     0x000021b8 (0x40019c)
     0x00002685 (0x4001f8)
     0x00004f65 (0x401120)
     0x00005187 (0x401120)
     0x000051c2 (0x40019c)
     0x000052be (0xffffffff)
     0x00005bab (0x246)
     0x000019c4 (0x400078)
Fatal fault in thread 0x0040019c! Aborting.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>