tests: mbox: mbox_api: Disable execution on RAM constrained devices.
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 <06b44f6fa9a97a2b955c2d2cd070279967447860@intel.com>

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