Refs #23778 -- Removed "Running tests outside the test runner" section.

As mentionned by @carljm, this is already covered in the following
"using other test frameworks" section.
Refs #23778 - Changed the section title.
Refs #23778 -- Simplifed testing scaffold structure
Refs #23778 -- Drastically simplified runtests.py
Refs #23778 -- Rewritten PYTHONPATH management paragraph

Removed setuptools reference and promote pip install -e as
the way to go instead of manually dealing with the PYTHONPATH.
Refs 23778 -- Simplified test_settings.py sub-section

More generic: a single test_settings.py file and no
reference to the DATABASES setting.
Refs #23778 -- Removed the urls.py subsection
Refs #23778 -- Changed pip install -e path

Using a path to the source you are working on is more explicit
that giving a git uri.
Refs #23778 -- Fixed some wording