tests: mbox: mbox_api: Disable execution on RAM constrained devices.

SRAM size is very small and so we cant fit the test-suite inside.
Hence disabling the execution on boards with very small RAM region.
Example: quark_d2000 & nucleo f030

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>