tests: add min_flash option for some tests

Following tests were failing on a microcontroller with 32KB flash:
    benchmark.application
    crypto.ecc_dh
    crypto.ecc_dsa
    test_build_ethernet
    test_build_sensors_a_m

The min_flash option has been added in the test case yaml files.

Signed-off-by: Diego Sueiro <diego.sueiro@gmail.com>