Added api tests for importing JSON feature

[IamEzio]

Fixes part of #2895
Note: This PR is a follow-through of PR #2968

This PR adds api tests for checking:

• JSON normalization code
• correct handling of unsupported JSON structures
• correct handling of invalid JSON structures

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  visible errors.

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[dmos62]

Approving, but #2968 needs to be merged first.