Use java-test-fixtures plugin

[mars885]

Incorporate the java-test-fixtures plugin to share common testing code across the project and delete commons-testing module, which acts as a replacement for it at this point.

Unfortunately, the plugin, at the moment of writing this issue, is only available for pure java and kotlin modules, meaning it cannot be used for android modules. There is a feature request on the Google Issue Tracker to support java-test-fixtures plugin.

A good resource about what text-fixtures-plugin is and what it can do can be found here.

[mars885]

https://youtrack.jetbrains.com/issue/KT-50667