Fix SSHManager cleanup exceptions during tests on NixOS

NixOS does not have ssh available within the build environment when running the tests, the following error is subsequently raised:

=============== 201 passed, 31 skipped, 27 deselected in 36.30s ================
An exception occured during cleanup, call the cleanup() method on targets yourself to handle exceptions explictly.
Error: {} [Errno 2] No such file or directory: 'ssh'
An exception occured during cleanup, call the cleanup() method on targets yourself to handle exceptions explictly.
Error: {} [Errno 2] No such file or directory: 'ssh'
An exception occured during cleanup, call the cleanup() method on targets yourself to handle exceptions explictly.
Error: {} child has vanished
Finished executing pytestCheckPhase

We should ensure that we cleanup test connections ourselves in the tests, so the global cleanup function doesn't catch those.