Devicetree: Devicetree Bindings: Add tests for new DT_ENUM_ macros

Test the new c-macros for dt enums. The new macros are already used in
the existing macros. As an example, DT_ENUM_IDX(node_id, prop) uses
DT_ENUM_IDX_BY_IDX(node_id, prop, 0) to get its result. However, this is
insufficient for testing the complete functionality of these macros.
Therefore, additional tests are added to make sure they work
appropriately for other indices besides 0.

Signed-off-by: Joel Hirsbrunner <jhirsbrunner@baumer.com>