Ignore some files/paths when running CS checks

- Recent changes to php-cs-fixer (see
  https://github.com/fabpot/PHP-CS-Fixer/issues/422) have led to errors being
  raised that should not. This change to the php-cs-fixer configuration places
  several new paths into the "do not test" list. They will be re-instated when
  the issues are fixed.
Merge branch 'hotfix/cs-overzealousness'

Fixes #6520