tests: net: Increase the stack size of frdm-k64f

The icmpv6, http_header_fields, mqtt_packet, dns_packet and
net utils tests were failing in frdm-k64f because of too small
stack.

Fixes #8171

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