Changed the way we test mock

Changed the way we test mock

After a long discussion as a result of https://dev.arvados.org/issues/17014#note-18
Tom Clegg and Nico Cesar agreed that TestCreateRemoteContainerRequestCheckRuntimeToken
was expecting the same sent json object in the mock

the process is
client -> zzzzz cluster -> zmock

and we expect to have a random runtime_token in the zmock cluster

Arvados-DCO-1.1-Signed-off-by: Nico Cesar <nico@curii.com>