Issue #3406460 by quietone, smustgrave: Fix spelling of array keys in tests

core/misc/cspell/dictionary.txt

...stem/tests/modules/ajax_forms_test/ajax_forms_test.module

.../ajax_forms_test/src/Form/AjaxFormsTestValidationForm.php

...ules/form_test/src/Form/FormTestOptionalContainerForm.php

...m/tests/modules/form_test/src/Form/FormTestSelectForm.php

...les/form_test/src/Form/FormTestVerticalTabsAccessForm.php

...ystem/tests/src/Functional/Form/ElementsContainerTest.php

core/modules/system/tests/src/Functional/Form/FormTest.php

.../FunctionalJavascriptTests/Ajax/AjaxFormPageCacheTest.php

.../FunctionalJavascriptTests/Ajax/ElementValidationTest.php