Add a test for `dcos auth list-providers`
Add a test for `dcos auth list-providers`

It tests for table outputs.

kensipe/dcos-cli,dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli