Issue #2551335 by alexpott, Wim Leers: Remove unnecessary SafeMarkup usage in tests and clear the static cache before every run

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2051467 by jhedstrom, dawehner, amateescu, Berdir, herom, Mile23: Expand and convert to phpunit tests for \Drupal\Component\Transliteration