Add automated tests

[Alkarex]

Please consider adding some (automated) tests, in particular to ensure compatibility with FreshRSS.
See example
Joedmin/xExtension-wallabag-button#1

A little detail is that on the main extensions repo, this extension is generating a warning (due to a limitation of our test tools), which would be nice to get rid of:

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  Line   third-party/8c042738017d75b31bcbc23d84cfb35d6315f3a7/comics/penny-arcade.php                                                                                                                                                        
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  :59    Inner named functions are not supported by PHPStan. Consider refactoring to an anonymous function, class method, or a top-level-defined function. See issue #165 (https://github.com/phpstan/phpstan/issues/165) for more details.  
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[giventofly]

Hi,

I currently don't have time to implement tests, but will keep that in mind for the future. If anyone is reading this and wants to do a PR for this, I can merge it

[Alkarex]

FreshRSS/Extensions#248