Add test to check that sealed unions are serialized the same way as legacy unions

[mpritham]

Before this PR

Add a small test to validate that the same Conjure union object is serialized the same way whether using the sealed classes implementation or the legacy implementation.

After this PR

==COMMIT_MSG==
Add test to check that sealed unions are serialized the same way as legacy unions
==COMMIT_MSG==

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Description

Add test to check that sealed unions are serialized the same way as legacy unions

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[mpritham]

Here is the test. The remainder of the code is modifying SimpleUnion in ete-service.yml to match the one in example-unions.yml

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💡 Improvements

• Add test to check that sealed unions are serialized the same way as legacy unions (#2752)

[kunalrkak]

@mpritham We have a bunch of these serde tests here, with the expected output hardcoded (since it should not change) - but if you want to add a object <> object test, that file feels like a better place for it!

[mpritham]

Sounds good. Moved.

[aldexis]

It's not immediately clear which one is the sealed interface and which one isn't. Should we add some verification that one is sealed and one isn't? (to avoid test refactors breaking our assumptions by making both sealed or both unsealed)

[mpritham]

I'm importing the SimpleUnion from the sealedunions package now