Added several tests based off missing code coverage

[mcassaniti]

This PR relates to #197.

I've added the following tests for Transport.build_session:

• Testing that a trusted CA path sets session.verify regardless of server_cert_validation
• Tested server_cert_validation=ignore code path
• Tested invalid server_cert_validation value
• Tested Kerberos delegation value as both valid and invalid string
• Tested not providing a username or password when they should be required
• Tested setting an invalid authentication mechanism
• Tested setting an invalid encryption setting

I've also added a Session.run_ps test which should test various code paths. The test returns an error, so the std_err and return code are also tested.

[coveralls]

Coverage increased (+6.6%) to 74.91% when pulling b04c90d on mcassaniti:new_tests into eb6a408 on diyan:master.

[coveralls]

Coverage increased (+6.6%) to 74.91% when pulling b04c90d on mcassaniti:new_tests into eb6a408 on diyan:master.

[coveralls]

Coverage increased (+7.7%) to 75.986% when pulling d76ad98 on mcassaniti:new_tests into eb6a408 on diyan:master.

[mcassaniti]

Thanks for taking a look. Much appreciated.

[badcure]

@mcassaniti Do you mind rebasing your project?

[badcure]

Whoops, looks like I changed test_transport is structured after you re-based. Sorry about that.

I will create another PR based on your branch and fix the conflicts, since I have already spent some time with the new file.