testsuite: group all test-related APIs under 'testing'

Before this change we had 2 top-level Doxygen entries for Testing APIs:

- Zephyr tests
- Zephyr testing suite

This patch creates a single top-level group to contain ZTest and FFF
extensions.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>