Implement fixture for `create_store`

[mih]

In the old code base this is a helper function from datalad.customremotes.ria_utils. It takes a IO abstraction and build a RIA store serverside.

We should not continue using this helper, and instead use a fixture.

This fixture can utilize a local directory to set such store up, if there is one. Otherwise it can use SSH-based commands. There is no point in forcing a test developer to deal with this complexity individually.

This issue becomes apparent in https://github.com/datalad/datalad-ria/pull/58

[adswa]

Implemented for

• [x] local: #97
• [x] ssh: #95
• [ ] http

[mih]

Fixed via #97