functional: make TestUnitSubmit() use cluster.WaitForNUnitFiles
functional: make TestUnitSubmit() use cluster.WaitForNUnitFiles

Improve TestUnitSubmit() like the following:
* use list-unit-files instead of list-units, to correctly verify
  submitted units.
* check that the output of list-unit-files contains the correct
  unit name, with help of a new helper cluster.WaitForNUnitFiles.
  That way the whole functional test could become less racy.

coreos/fleet,dalbani/fleet,endocode/fleet,jonboulle/fleet,giantswarm/fleet,jonboulle/fleet,jonboulle/fleet,endocode/fleet,giantswarm/fleet,giantswarm/fleet,dalbani/fleet,dongsupark/fleet,dongsupark/fleet,endocode/fleet,dalbani/fleet,coreos/fleet,dongsupark/fleet,coreos/fleet