[meta] Consolidate phpunit_example and simpletest_example into testing_example

Problem/Motivation

We should not segregate the test examples based on the test runner. We should, instead give examples of all the different test techniques and types in one place.

Proposed resolution

Fold both phpunit_example and simpletest_example into testing_example, one each in child issues.

Remaining tasks

Follow-ups for phpunit_example and simpletest_example.

User interface changes

API changes

Data model changes