Flaky echo service test

[daniel-sanche]

We have been seeing flakes in the showcase tests:

FAILED tests/system/test_lro.py::test_lro[rest] - google.api_core.exceptions.NotFound: 404 GET
http://localhost:7469/v1beta1/operations/google.showcase.v1beta1.Echo/Wait/CgsIu/NyAYQ2IfcLBo8CjpUaGUgaGFpbCBpbiBXYWxlcyBmYWxscyBtYWlubHkgb24gdGhlIHNuYWlscy4uLmV2ZW50dWFsbHku:
Operation "operations/google.showcase.v1beta1.Echo/Wait/CgsIu/NyAYQ2IfcLBo8CjpUaGUgaGFpbCBpbiBXYWxlcyBmYWxscyBtYWlubHkgb24gdGhlIHNuYWlscy4uLmV2ZW50dWFsbHku"
not found.