Change header title

Aligns better with the previous header which is "Installing Salt for
development".
Remove duplicate "here"
Unify documentation regarding the test suite

There were a whole lot of information regarding running the test suite
in the hacking file, while there are a separate tests section of the
documentation. I attempt to slim this down a bit and give a helpful
pointer to the full section in order to avoid duplicated texts.
Remove section title

Since this is a very short chapter, it didn't really add any value.
Add link to SaltStack Inc blog
Link to the hacking doc instead of including it all

Serves the purpose and is easier to manage.
Merge pull request #8407 from holmboe/doc-test-changes

Doc changes (mostly regarding the test suite)