As a user, I can query Test issues of a specific Test Plan using a JQL function

Xray must provide a JQL function for filtering Tests in a Specific Test Plan.

issuetype = "Test" and issue in testPlanTests(test_key, status, environment)

Required parameters are: test_key

The "status" parameter is optional and allows to filter Test issues in a specific Plan with the specified execution status.
If the "status" parameter is present, users might also pass the "environment" parameter. If this parameter is filled Xray will provide all Tests in a Test Plan that are in the specified "status" and for the specified "environment".