As a user, I can browse, add and remove Test Executions from a Test Plan using the REST API

GET /testplan/{testPlanKey}/testexecution - returns all Test Executions associated with the Test Plan
POST /testplan/{testPlanKey}/testexecution - Adds or removes Test Executions associations to the Test Plan
DELETE /testplan/{testPlanKey}/testexecution/{testExecKey} - Removes the test execution association to the Test Plan