Revert "Issue #2171683 by sun, tstoeckler, larowlan: Remove all Simpletest overrides and rely on native multi-site functionality instead."

This reverts commit 91db25eba49dfedb57e9abb4ce1d1cb0bc146ebc. Broke web
tests.
Issue #2081963 by mr.baileys, dawehner: Shortcut add/remove link on titles don't work on route only pages.
Issue #2188595 by alexpott, jibran: Create a ConfigManager to be able to remove config.inc.
Issue #2090509 by yched, swentel, Wim Leers: Optimize entity_get_render_display() for the case of 'multiple entity view'.
Issue #2171683 by sun, alexpott, tstoeckler, larowlan: Remove all Simpletest overrides and rely on native multi-site functionality instead.
Issue #1366710 by chx: Remove chx from MAINTAINERS.txt (except migrate).
Issue #2190643 by Berdir, amateescu, Xano, yched: Serializing the container is a very very bad idea, let's prevent it?.
Issue #2192809 by andrejsmuzikovs: Drupal.formatString() doesn't properly replace all placeholders.
Issue #2193023 by Wim Leers, dawehner: EditorXssFilter/StandardTest::dataset #25 fails on php 5.4.
Issue #2030657 by ceng, marvin_B8, daffie: Expand Shortcut with methods.
Issue #850728 by tim.plunkett, bleen18, toco, kscheirer: Bartik: template.php cleanups.
Issue #2181863 by karthik kumar bodu: Xpath method on \Drupal\simpletest\WebTestBase has an undocumented  parameter.
Issue #2182315 by joachim: Simpletest verbose output doesn't show end result of a drupalPostAjaxForm().
Issue #2045931 by Sam152, joelpittet, LinL, Sutharsan, adsw12: Replace all module_exists() deprecated function calls.
Issue #2094335 by andypost: Statistics.php needs a lower bootstrap level.
Issue #922696 by mgifford, EvanDonovan, LewisNyman, stpaultim, jensimmons: New methods, standards for describing core themes on themes admin page (was Theme screenshots do not have adequate alt text...).
Issue #2183419 by kgoel: In StylePluginBase Rename Views properties to core standards.
Issue #2186847 by InternetDevels: Remove duplicate edit-form key in Drupal\aggregator\Entity\Feed.
Issue #2185689 by yched: Remove stale entries in field / field instance schemas.
Issue #2110951 by Xano: Remove hook_forms().