Add functional tests

The extension has no functional tests, so nothing verifies that it can be installed on XWiki and that the macro and its administration work in a running wiki. Writing them found PLANTUML-33 (the macro can't be installed on XWiki < 16.4) and PLANTUML-19 (the default PlantUML server makes all macros fail).

Add Docker-based functional tests verifying:

• the diagram generation by the embedded PlantUML library, in the PNG, SVG and TXT formats, with the title and type parameters
• that a PlantUML server whose domain isn't trusted is refused
• the diagram generation by a remote PlantUML server (running in a Docker container), set with the server parameter or in the wiki administration