Clarify ActiveRecord testing guidelines [ci skip]

1. Specify that you need to create the test databases, and that no special
   Rails command needs to be run to do that.
2. Although the underscore style of `rake test_mysql` works, make the
   documentation of running the tests in RUNNING_UNIT_TESTS.rdoc
   consistent with the "Contributing..." guide.
3. Promote "Testing Active Record" to not be a subsection of
   "Running a Single Test," since it doesn't make sense as a subsection
   of that.
It works, but the contributing doc says to use test:mysql (which also works). 

I don't guess it matters which style you use, but if someone is new to contributing I think it's best to not confuse them.