Create Test for sftp

[atomic111]

create the tests for this commit dev-sec/chef-ssh-hardening#111

[rndmh3ro]

I tried to add a test for sftp here: https://github.com/rndmh3ro/tests-ssh-hardening/blob/sftp/controls/sshd_spec.rb#L458-L465

But so far my attempts in using the sshd resource failed. These do not work:

  describe sshd_config do
    its('Subsystem sftp') { should eq nil }
end

  describe sshd_config do
    its('Subsystem sftp') { should_not exist }
end

  describe sshd_config do
    its('Subsystem') { should_not include('sftp') }
end

Is it possible with the sshd resource or should I just use the file module with content?