* Emphasize use-cases first -- and then drill-down to tools/specifics.
* Add a placeholder section for extension PRs
* Re-style and copy-edit the section for core PRs

This is a significant revision of the PHPUnit documentation. This specifically:

* Adds concrete examples of writing tests in various styles (`CiviUnitTestCase`, `CiviEndToEndTestCase`, `civix`)
* Adds reference documentation for classes/traits/interfaces provided by Civi's test library
* Imports key information from Testapalooza docs (#389)

Re-arrange CI doc as per some comments from Tim and link to other docs on test suites rather than replicating

The purpose of the intro paragraph is to prep the reader to see several
different test types/tools.  So it...  should probably use the word "test"
somewhere.

I'd like to be able to merge the general, incremental improvements for the
"Testing" section, but these two new pages aren't yet written.  Comment them
out.

 * Provide a better visualization of the the different test suites
 * Migrate setup instructions from https://github.com/civicrm/civicrm-core/blob/master/tests/README.md

There are multiple tools for testing in JS. We'll want separate pages for each.

The second statement doesn't make much sense without the first statement.
Put them together.

Running civilint is important for any/all code.  Seems a bit arbitrary
emphasizing this point specifically for tests.