Update testing guide to include starting browser containers

[rgraber]

In order to run test_js_run you must first bring up either Firefox or Chrome in a container, but the docs are not clear about this.

A/C

• Update https://github.com/openedx/edx-platform/blob/master/docs/guides/testing/testing.rst#running-javascript-unit-tests to include the necessity to bring up the Firefox or Chrome container
• Find and update any other docs that refer to how to run js tests