CKEditorPluginManagerTest fails because plugins are discovered in slightly different order

#1890502: WYSIWYG: Add CKEditor module to core was committed with pretty solid test coverage; one of the things it tests are the available CKEditor plugins out-of-the-box. This uses PluginManagerBase::getDefinitions(), which does not provide any guarantees on the order in which the definitions are returned. For some unknown reason, it used to pass just fine at #1890502: WYSIWYG: Add CKEditor module to core, but now, that's no longer the case. (I don't know what the root cause is, but clearly plugin discovery is either stateful or non-deterministic.)

Patch test failures at:

• #1847596-58: Remove Taxonomy term reference field in favor of Entity reference
• #1905424-4: Update CKEditor library (a JS-only patch, which *cannot make tests fail*!)
• #1898844: No test coverage for hook_editor_js_settings_alter()
• …