test: Close ssh's stdout properly
test: Close ssh's stdout properly

This avoids a warning when running the tests with Python 3:

    bots/machine/testvm.py:348: ResourceWarning: unclosed file <_io.BufferedReader name=13>
      self.ssh_process = None

Closes  #8540
martinpitt/cockpit,deryni/cockpit,cockpituous/cockpit,stefwalter/cockpit,stefwalter/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mareklibra/cockpit,stefwalter/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit,garrett/cockpit,mvollmer/cockpit,stefwalter/cockpit,andreasn/cockpit,deryni/cockpit,martinpitt/cockpit,cockpituous/cockpit,martinpitt/cockpit,cockpituous/cockpit,deryni/cockpit,mvollmer/cockpit,SotolitoLabs/cockpit,cockpituous/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,garrett/cockpit,andreasn/cockpit,martinpitt/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,cockpit-project/cockpit,andreasn/cockpit,mareklibra/cockpit,garrett/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit,mareklibra/cockpit,garrett/cockpit,deryni/cockpit,moraleslazaro/cockpit,andreasn/cockpit,andreasn/cockpit,mvollmer/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,mareklibra/cockpit,cockpit-project/cockpit,SotolitoLabs/cockpit,deryni/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,cockpit-project/cockpit,mvollmer/cockpit,andreasn/cockpit,andreasn/cockpit,garrett/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,mareklibra/cockpit,stefwalter/cockpit,mareklibra/cockpit