tests: settings: fcb: increase ztest stack size to 2kB

Tests are passing now on `nrf52840dk_nrf52840` platform, just because ARM
MPU is disabled and stack overflow is not detected. When running those
tests on `qemu_x86` there is following error during runtime:

  START - test_config_compress_reset
  E: Double Fault
  E: EAX: 0x0011c1c8, EBX: 0x00000002, ECX: 0x00000002, EDX: 0x0000100a
  E: ESI: 0x0000000a, EDI: 0x0021f370, EBP: 0x0021f000, ESP: 0x0021f000
  E: EFLAGS: 0x00000202 CS: 0x0008 CR3: 0x0022b000
  E: call trace:
  E: EIP: 0x001075be
  E:      corrupted? (bp=0x21f000)
  E: >>> ZEPHYR FATAL ERROR 2: Stack overflow on CPU 0
  E: Current thread: 0x11b140 (test_config_compress_reset)
  E: Halting system

Same happens on `nrf52840dk_nrf52840` when ARM MPU is enabled:

  START - test_config_compress_reset
  E: ***** MPU FAULT *****
  E:   Stacking error (context area might be not valid)
  E:   Data Access Violation
  E:   MMFAR Address: 0x20008cb8
  E: r0/a1:  0x39beca54  r1/a2:  0x9abe9566  r2/a3:  0xe333d994
  E: r3/a4:  0x41387fc2 r12/ip:  0x7c2e5c00 r14/lr:  0xdc5006d9
  E:  xpsr:  0x81000000
  E: Faulting instruction address (r15/pc): 0x0000b50e
  E: >>> ZEPHYR FATAL ERROR 2: Stack overflow on CPU 0
  E: Current thread: 0x200001d8 (test_config_compress_reset)
  E: Halting system

Increase ZTEST stack size to 2kB to prevent stack overflow.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>