Make tests pass with new export signature

The plugin now provides a default and named exports. So the tests now
must explicitly test the default export.

Author: e-tobi

src/__tests__/plugin.spec.js

@@ -1,6 +1,6 @@
 import { mount } from '@vue/test-utils';
 import { createApp } from 'vue';
-import * as Main from '../plugin';
+import Main from '../plugin';
 import io from '../__mocks__/socket.io-client';
 
 it('should be vue plugin (is an object with `install` method)', () => {