Issue #1960574 by swentel, smiletrl: Add more tests for Field config import.

...es/field/lib/Drupal/field/Tests/FieldImportCreateTest.php

...es/field/lib/Drupal/field/Tests/FieldImportDeleteTest.php

...ield_test_config/config/field.field.field_test_import.yml

...ld_test_config/config/field.field.field_test_import_2.yml

...ld.instance.test_entity.test_bundle.field_test_import.yml

....instance.test_entity.test_bundle.field_test_import_2.yml

...nstance.test_entity.test_bundle_2.field_test_import_2.yml

...onfig/staging/field.field.field_test_import_staging_2.yml

...e.test_entity.test_bundle.field_test_import_staging_2.yml

...test_entity.test_bundle_2.field_test_import_staging_2.yml