RSpec setup: proper waiting for HGitaly

We couldn't do it right at the time HGitaly appeared because
it wasn't Unix Domain socket capable (and it didn't look to
be critical, given the setup startup times of other components,
hence we did not make the effort to adapt the `wait` method to
TCP sockets).

Nowadays, doing it is a minimal effort, so here we are.

branch : heptapod
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