Fix flaky tests

[campoy]

Currently there's tests that fail occasionally, I think this might be because of dependence on the network to fetch resources.

Example from a job:

FAIL: <autogenerated>:26: ReceivePackSuite.TestAdvertisedReferencesNotExists
/home/travis/gopath/src/gopkg.in/src-d/go-git.v4/plumbing/transport/test/receive_pack.go:45:
    c.Assert(err, Equals, transport.ErrRepositoryNotFound)
... obtained *url.Error = &url.Error{Op:"Get", URL:"http://localhost:37003/non-existent.git/info/refs?service=git-receive-pack", Err:(*net.OpError)(0xc420010550)} ("Get http://localhost:37003/non-existent.git/info/refs?service=git-receive-pack: dial tcp 127.0.0.1:37003: getsockopt: connection refused")
... expected *errors.errorString = &errors.errorString{s:"repository not found"} ("repository not found")

I wonder if this should be fixed by faking the network (stubbing http.Get) or by simply retrying failing tests.