tests: net: lib: dns: dns_addremove: Increase stack size for FRDM-K64F

CONFIG_MAIN_STACK_SIZE was originally 1344 in this test.  The following
commit removed setting the stack size in the test:

commit 7b3cd7d3
Author: Cami Carballo <cami.carballo@intel.com>
Date:   Fri Jul 26 10:59:16 2019 -0400

    tests: net: increase stack size

However getting the default stack size on FRDM-K64F of 512 causes the
following panic:

FATAL: ***** BUS FAULT *****
FATAL:   Stacking error
FATAL:   Imprecise data bus error
FATAL:   NXP MPU error, port 3
FATAL:     Mode: Supervisor, Data Address: 0x20002ff0
FATAL:     Type: Write, Master: 0, Regions: 0x8200
FATAL: r0/a1:  0x00000000  r1/a2:  0x00000000  r2/a3:  0x00000000
FATAL: r3/a4:  0x00000000 r12/ip:  0x00000000 r14/lr:  0x00000000
FATAL:  xpsr:  0x200002f4
FATAL: Faulting instruction address (r15/pc): 0x00000000
FATAL: >>> ZEPHYR FATAL ERROR 2: Stack overflow
FATAL: Current thread: 0x2000182c (main)
FATAL: Halting system

Bump stack back to original 1344 fixes the issue.

Fixes #18292

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>