ability to suppress prints in language tests, unless they fail

[jmesserly]

Some of the language tests print a lot of stuff. For now I'm commenting out the print because it goes to console.log in browser, and Karma routes it to your the terminal. Ideally this would only print if the test fails

[vsmenon]

This might be what we want: https://www.npmjs.com/package/karma-logcapture-reporter