test: Move run-tests serial machine assignment into Test method
test: Move run-tests serial machine assignment into Test method

This avoids poking around in Test.command from outside, avoids some
duplicate code, and paves the way for making the scheduler more dynamic:
We don't need to assign a machine to a Test on construction any more, it
just needs to be done before calling .start().

martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit