Uninstall tests: prepare to use JDGrimes/wp-plugin-uninstall-tester

See #16
Squashed 'tests/phpunit/includes/uninstall/' content from commit 8bf3958

git-subtree-dir: tests/phpunit/includes/uninstall
git-subtree-split: 8bf3958f714e01c9cc045a09d4edc760a9111d27
Merge commit '2a692ed9ad5d0ad1f94fda59259e17c9408b3c99' as 'tests/phpunit/includes/uninstall'
Squashed 'tests/phpunit/includes/uninstall/' changes from 8bf3958..ae183d4

ae183d4 Plugin install: use wp-settings.php instead of wp-load.php

git-subtree-dir: tests/phpunit/includes/uninstall
git-subtree-split: ae183d4aee331b140aa3be92e8266d414911e26c
Update the uninstall tester from JDGrimes/wp-plugin-uninstall-tester
Unit tests: remove the uninstall testcase class, as it is no longer used

See #16
Uninstall: avoid a fatal error in the points component by including dependencies

Fixes #16