tests: lib: ringbuffer: Fixing tests for various platforms

Stress test that was validating thread safeness was configured to use
higher sys tick rate and targeting qemu_x86 platform. However, it was
also build for other platforms and was failing. Fixing it by extracting
stress tests to separate configuration that is run only on qemu_x86.

Modified the test to fail when number of preemptions is less than
expected only on qemu_x86.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>