Summary

This PR can be summarized in the following changelog entry:

• PHPUnit: add UninstallationTest

Relevant technical choices

• Adds a MethodSpy helper for making assertions about method calls which will be useful in other tests as well.

Checklist:

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).