tests: net: Increase stack sizes for coverage testing

We need to increase the stack sizes if user enables CONFIG_COVERAGE
option. You can test this by this command

    sanitycheck --coverage -p qemu_x86 -T tests/net/udp \
                -T tests/net/mgmt  -T tests/net/mld

This is partial fix for issues described in #17323

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