Testing docs and helpers

[spalger]

This pr creates several helper tasks for running the karma tasks, and documents them in contributing.md.

From contributing.md changes:

Debugging test failures

The standard npm run test task runs several sub tasks and can take several minutes to complete, making debugging failures pretty painful. In order to ease the pain specifialized tasks provide alternate methods for running the tests.

npm run test:quick

Runs both server and browser tests, but skips linting

npm run test:server or npm run test:browser

Runs the tests for just the server or browser

npm run test:dev

Initializes an environment for debugging the browser tests. Includes an dedicated instance of the kibana server for building the test bundle, and a karma server. When running this task the build is optimized for the first time and then a karma-owned instance of the browser is opened. Click the "debug" button to open a new tab that executes the unit tests.

[jbudz]

specialized

[jbudz]

Commented on a typo, otherwise looks good. Sending to @rashidkpc second looks.

[w33ble]

maybe using test:quick here would make sense, so we only need to add new test types in 1 place?

[w33ble]

All the scripts seem to work. I like this change 💎

@spalger one quick comment there, but it's honestly good to merge as-is too. Fix, or don't, then click that beautiful green button