Create PHPUnit tests for this plugin

[mhoffrog]

Unit tests for this plugin will speed up testing - and in consequence - the accepting of pull requests.

The implementation must follow the guide line at Unit testing for DokuWiki

To get started one may lookup existing implementations of unit testing at plugins like:

• captcha Plugin for DokuWiki - Unit tests
• upgrade Plugin for DokuWiki - Unit tests
• edit table Plugin for DokuWiki - Unit tests