Is it possible to write a test for the case fixed here?

Yes it is possible but I think it is not needed. Because this PR only removes unnecessary code

        if (static::isEnum($newColumn)) {
            $newColumn->dbType = 'enum'; // TODO this is concretely not correct
        }

rest all changes are just of refactoring and not related to this PR.

Also existing test covers this PR:

• yii2-openapi/tests/unit/EnumTest.php

  Line 240 in c4b4cf6

  'connection' => 'string',

• yii2-openapi/tests/specs/enum/fresh/mysql/app/migrations_mysql_db/m200000_000000_create_table_editcolumns.php

  Line 13 in 8250c09

  'connection' => 'enum("WIRED", "WIRELESS") NOT NULL DEFAULT \'WIRED\'',

• yii2-openapi/tests/specs/enum/fresh/mysql/enum.yaml

  Line 83 in d13479f

  connection:

so I think no new tests are needed

Thank you!