Add test cases for backed enum implementing interface

In the Enumeration RFC, it states Backed Enums can implement
interfaces, but it was not clear where the backed Enum type
would n

Add test cases for backed enum implementing interface

In the Enumeration RFC, it states Backed Enums can implement
interfaces, but it was not clear where the backed Enum type
would need to be placed in such situation.

This commit adds a test case for these scenarios.

Signed-off-by: Agustin Gomes <me@agustingomes.com>

Closes GH-7593.

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