Uninstall: update routines and tests for multisite

See #1
Squashed 'tests/phpunit/includes/uninstall/' changes from 427d1c5..497e387

497e387 Support multisite testing.

git-subtree-dir: tests/phpunit/includes/uninstall
git-subtree-split: 497e3878a32a1e29e9c316ebb00401476c18f299
Merge commit 'ab5a646690ef15a7c0d98836acef59c8fa20752b'

Updates the uninstall tester for the multisite tests. See 0807dbc5f507f26477ed0217df2c207099aa9d1b