dm: test: Add tests for get/find uclass devices

This commit introduces simple tests for functions:
- uclass_find_first_device()
- uclass_find_next_device()
- uclass_first_device()
- uclass_next_device()

Tests added by this commit:
- Test: dm_test_uclass_devices_find:
  * call uclass_find_first_device(), then check if: (dev != NULL), (ret == 0)
  * for the rest devices, call uclass_find_next_device() and do the same check

- Test: dm_test_uclass_devices_get:
  * call uclass_first_device(), then check if:
    -- (dev != NULL), (ret == 0), device_active()
  * for the rest devices, call uclass_next_device() and do the same check

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>