Dedup devref on unit tests

We already have a document on how to run unit tests, so
development.environment should just point to that document instead of
trying to summarize it.  Keeping this information in one less places
makes it easier to make sure the page is up to date (which
development.environment is not -- no mention of tox).

Change-Id: I9aecf439b6145b035b3457559ddb187058f5057b
Merge "Dedup devref on unit tests"