Add a test to a group
Add an existing test to a test group. Returns 201 when a new membership row was created, or 200 with added: false when the test was already a member (idempotent).
Add an existing test to a test group. Returns 201 when a new membership row was created, or 200 with added: false when the test was already a member (idempotent).
Test was already a member; no change