[bug 891549] Document LiveServerTestCase tests

I copy and pasted the docs from Kitsune. If they are right there, they're
probably right here.
Update testing docs re: collectstatic and compress_assets

LiveServerTestCase spins up a server with DEBUG=False so we need to
run collectstatic and compress_assets commands before running tests.
This clarifies that in the docs.

Plus this fixes some other testing docs issues from when I copy
and pasted it from Kitsune docs.