Issue #3619689 by luissousa21: module_installer_config_test.schema.yml is...

Closes #3619689

Adds the missing third-party settings schema for the test_config_type config entity.

Without it, the [%parent.%parent.%type].third_party.[%key] item type has nothing to resolve to, and getFallbackName() collapses the wildcards all the way back to module_installer_config_test.type.* the entity's own schema. So any third-party setting fails validation.

Also fixes config_export, which listed label while both the label entity key and the schema use name.

Kernel test added. Test-only fails with both errors, with the fix it passes.