Issue #2371843 by alexpott: Add event listener to check schema on config save.

core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php

core/lib/Drupal/Core/Config/Testing/ConfigSchemaChecker.php

core/modules/config/src/Tests/SchemaCheckTestTrait.php

core/modules/config/src/Tests/SchemaCheckTraitTest.php

core/modules/config/src/Tests/SchemaConfigListenerTest.php

.../modules/config/src/Tests/SchemaConfigListenerWebTest.php

.../modules/config/tests/config_test/config_test.routing.yml

...config/tests/config_test/src/SchemaListenerController.php

core/modules/simpletest/src/KernelTestBase.php

core/modules/simpletest/src/TestBase.php

core/modules/simpletest/src/WebTestBase.php

core/modules/system/src/Tests/Menu/MenuTreeStorageTest.php