test-lfs: avoid a bashism when spawning the server
For zsh, this &> call is read as "& >", this spin the process without
redirection. As a result the server grab stdout, which does not get closed at the
end of the test. As a result `run-tests.py` hang there forever, waiting for the
the stream for be close. (which is probably as issue on its own).
Differential Revision: https://phab.mercurial-scm.org/D10396
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