tests: net: Increase the size of main stack

This avoids this crashing in qemu_x86

***** Stack Check Fail! *****
Current thread ID = 0x004015e0
Faulting segment:address = 0x0008:0x00003593
eax: 0x0041e930, ebx: 0x00000000, ecx: 0x004170bc, edx: 0x0000dff8
esi: 0xffffffff, edi: 0x0041e930, ebp: 0x00417010, esp: 0x00416ffc
eflags: 0x216
Fatal fault in essential thread! Spinning...
Terminate emulator due to fatal kernel error

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>