feat: add unit test to check for valid JSON object returned in API

[DevinWalker]

Feature Request

This is a request for a simple unit test to check for valid JSON results returned from the various Give Core API endpoints as a result of this issue: #4071 which resulted in hours of debugging Zapier to no avail in https://github.com/impress-org/give-zapier/issues/38

User Story

As a developer I want to ensure the Give API always returns valid JSON so that my integrations continue working as expected when the plugin updates.

Possible Solution

Add unit test for results returned from API

Related

#4071 and Zapier issue: https://github.com/impress-org/give-zapier/issues/38

Acceptance Criteria

• Before Give is released a unit test is ran which tests the validity of the API result formats