rec: Don't stop the reactor in the regression tests

rec: Don't stop the reactor in the regression tests

So, there is one and only one global reactor for every test suites,
ever, and you can't restart it if you stop it (see the awful
`ReactorNotRestartable` exception).
That doesn't prevent you from adding new ports to listen on to the
existing one, so let's just start if it isn't already running and
never stop it.

(cherry picked from commit fbfaa4a7ee59829cbf2d3765a3a967edb823bb39)