Reset RepoStore before each test

Reset RepoStore before each test.

---

As an example, the RepoSidebar test failure here was caused by the store being in a awkward state(RepoStore.openedFiles = [undefined]) after other tests ran.

The test completed successfully on its own (the store default is RepoStore.openedFiles = []), and was patched over with this store reset, https://gitlab.com/gitlab-org/gitlab-ce/commit/d9a7aadd6d828680d652d71b0e0664f7550d0882