Add tests of command line functionality

[bdice]

Tests are needed to cover options to the CLI run command like ./dashboard.py run --debug --host 0.0.0.0 --port 10099. Debug mode was broken and fixed in #152 but neither @cbkerr nor I could determine a good way to test the CLI, because it launches a server. I tried briefly using multiprocessing and failed. I also read a bit about Flask.test_cli_runner but that didn't seem like the right thing.