tests: Add vcsbackend_stub as a new fixture

Corresponding to backend_stub.

rhodecode/tests/plugin.py

@@ -719,6 +719,16 b' def vcsbackend_random(vcsbackend_git):'
     return vcsbackend_git
 
 
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+def vcsbackend_stub(vcsbackend_git):
+    """
+    Use this to express that your test just needs a stub of a vcsbackend.
+
+    Plan is to eventually implement an in-memory stub to speed tests up.
+    """
+    return vcsbackend_git
+
+
 class VcsBackend(object):
     """
     Represents the test configuration for one supported vcs backend.