Merge tests repo

[DBX12]

I moved the tests from the separate directory to this repository so they can be cloned altogether as discussed here. (Ok, discussion is a strong word, it was talked about in two comments :D )

I can squash the commits if you want (or you do a squash merge), I kept all of them to make retracing my steps easier.

I hope I got the github workflow right, that was my first time working with them. And as always, I would greatly appreciate the hacktoberfest-accepted label on the PR :)

[DBX12]

As written above, I'm not "fluent" in github workflows. As far as I can judge this is because fxp/composer-asset-plugin is missing, would adding composer global require "fxp/composer-asset-plugin:~1.3" in the Install dependencies step fix the issue?

[samdark]

fxp/composer-asset-plugin should not be required for current Yii 2 apps.

[samdark]

https://github.com/yiisoft/yii2-app-advanced/blob/master/composer.json#L39 - this should help.

[DBX12]

Copying the .gitignore contents first and thinking about it second was not the smartest move 😅

[samdark]

👍