tests: application_development: add new test for ram-based isr

This commit introduces the new test ram_context_for_isr that shows
how to configure an application (prj.conf, CMakeLists.txt, etc.) in
order to enable full ISR execution with a RAM context.

It resolves the issue of flash latency affecting real-time constraints
during ISR execution.

Signed-off-by: Martin Hoff <martin.hoff@silabs.com>