Add tests for proto accesses on unions.
Add tests for proto accesses on unions.

These would've caught the issues that caused the rollback of []and also add some additional coverage.

GITHUB_BREAKING_CHANGES=none

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187042602

google/closure-templates,google/closure-templates,yext/closure-templates,Medium/closure-templates,yext/closure-templates,google/closure-templates,Medium/closure-templates,Medium/closure-templates,yext/closure-templates,google/closure-templates,yext/closure-templates,google/closure-templates,Medium/closure-templates,Medium/closure-templates