[Plugins] Add dependency for cordova-plugin-test-framework in /tests

Currently you have to manually also install https://github.com/apache/cordova-plugin-test-framework if you want to run the tests via `cdvtests/index.html` when adding the plugin tests.

This could probably be avoided by adding

```

```

to each `tests/plugin.xml` (Example: https://github.com/apache/cordova-plugin-camera/blob/master/tests/plugin.xml).