tests: cmake: Add zephyr_get

As `zephyr_get()` grows in complexity and significance, it must continue
working as expected. Unfortunately, it has a few minor bugs already, but
they will be taken care of in the next few commits. Before that, though,
let's go ahead and add a test suite for this function.

For now, some tests will pass by using incorrect values, annotated with
the `# BUG` comments; they will be removed in later patches. The purpose
of having them now is to make it easier to see what's about to be fixed.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>