Allow Env constructor to accept a remote instance
Implement basic Testing::MockRemote object
Implement Testing::MockRemote#execute
Implement rspec TestEnvHelpers:

  Allow use of our test environment, with a mocked remote and convenient
methods to write more concise expectations.

  #env, builds and returns a memoized test environment (with mocked
  remote)

  #output, returns env output as a string

  #remote_fs, returns env remote fs

  #expect_execution, setup a remote command execution expectation
Merge branch 'testing-testenv'

  Integrate a test environment, helping to reduce or isolate code
dealing with net-ssh or net-sftp dependencies setup.