Fix TestUserCreateBranch_successful writing into shared repository

TestUserCreateBranch_successful is setting up a shared repository in the tests. It then proceeds to concurrently write to the shared repository in the subtests. This leads to indeterministic results as the branch may or may not be already created by one of the other subtests. Fix this by running each of the tests agains their own repository. As all tests are now running agains their own repositories, we know longer need to delete the created branch at the end of the test.