dm: test: Add tests for get/find uclass's device by name

This commit introduces simple tests for functions:
- uclass_find_device_by_name()
- uclass_get_device_by_name()

Tests added by this commit:
- Test: dm_test_uclass_devices_find_by_name: for uclass id: UCLASS_TEST_FDT
 * get uclass's devices by uclass_find_first/next_device() each as 'testdev',
 * for each returned device, call: uclass_find_device_by_name(),
   with previously returned device's name as an argument ('testdev->name').
 * for the found device ('founddev') check if:
   * founddev != NULL
   * testdev == founddev
   * testdev->name == founddev->name (by strcmp)

- Test: dm_test_uclass_devices_get_by_name: for uclass id: UCLASS_TEST_FDT
 * get uclass's devices by uclass_get_first/next_device() each as 'testdev',
 * for each returned device, call: uclass_get_device_by_name(),
   with previously returned device's name as an argument ('testdev->name').
 * for the found device ('founddev') check if:
   * founddev != NULL
   * founddev is active
   * testdev == founddev
   * testdev->name == founddev->name (by strcmp)

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