Introduce remote fixture

[pared]

In many places in the tests we create local remote using tmp_path_factory (can be found by looking for RemoteConfig usages). And set it up "by hand". Introducing remote fixture should let us get rid of this boilerplate code. Few notes:

• consider how to handle non-local remotes, and if we should create such fixture for each kind of remote
• in erepo, we currently set remote to point to its cache, we should probably reconsider that and prepare separate remote for it when necessary

[efiop]

We now have multiple remote fixtures as well as local_remote fixture. Closing.