tests/net: utils: increase min RAM requirement to 24K

Min RAM requirement for tests/net/utils was 16K,
hence run on nucleo_l073rz.
Though, build size is a little more than 20K which
overflows board RAM.

Increase min RAM requirement for this test to 24K

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>