tests: settings: file: increase main stack size to 2kB

Running tests on `nrf52840dk_nrf52840` with ARM MPU enabled results in
following error:

  Running TESTSUITE settings_config_fs
  ===================================================================
  I: LittleFS version 2.5, disk version 2.0
  I: FS at mx25r6435f@0:0x0 is 16 0x1000-byte blocks with 512 cycle
  I: sizes: rd 16 ; pr 16 ; ca 64 ; la 32
  E: WEST_TOPDIR/modules/fs/littlefs/lfs.c:1234: Corrupted dir pair at \
     {0x0, 0x1}
  W: can't mount (LFS -84); formatting
  E: ***** MPU FAULT *****
  E:   Stacking error (context area might be not valid)
  E:   Data Access Violation
  E:   MMFAR Address: 0x20001f78
  E: r0/a1:  0x00000000  r1/a2:  0x00000000  r2/a3:  0x00000000
  E: r3/a4:  0x0000b461 r12/ip:  0x00000000 r14/lr:  0x0000da8d
  E:  xpsr:  0x00003800
  E: Faulting instruction address (r15/pc): 0x00000000
  E: >>> ZEPHYR FATAL ERROR 2: Stack overflow on CPU 0
  E: Current thread: 0x20000468 (main)
  E: Halting system

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

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