This really depends on a lot of things. If you'll be using a lot of similar steps between all the API tests they might benefit from being in a single repo. It also matters who will be maintaining the tests. If the developers will be maintaining the tests and/or utilizing them during development then it makes sense to keep them in the same repository as the code they are testing. Otherwise it's up to you. It really would be ideal for developers to contribute to the tests since their changes will be breaking tests left and right.