tests: add `min_ram` for heap-using testcases

Two testcases have an implicit minimum memory requirement due to heap usage
and are failing with `ENOMEM` for some platforms:
* portability.xsi.single_process.newlib
* kernel.message_queue.usage

This commit filters out platform that can't satisfy this requirement.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>