fix: D018_01/03 stub the forwarded create reply like sibling D018_02

D018_01 (loop detection) and D018_03 (Via-header forwarding) register an inclusive CSR and POST an entity, but never stub a reply for the forwarded create. On a broker that reports an unanswered inclusive forward as a partial success, the create then returns 207 and the test fails at setup, never reaching its loop-detection (508) / Via-header assertion. Sibling D018_02 already stubs the reply and passes.

Add the matching Set Stub Reply POST <path> 201 before Create Entity: /ngsi-ld/v1/entities for D018_01 (default endpoint), /broker1/ngsi-ld/v1/entities for D018_03 (endpoint=/broker1). With the stub the create + 201 check pass and the tests proceed exactly like D018_02.

Documented as testsuite-doubts #74 (closed). Whether a failed inclusive-write forward should yield 207 at all is a separate spec ambiguity (spec-doubts-2 #90).