This refactoring of test_resource_manager is also meant as proposal to reduce the environmental and internal dependencies in the ocrd-modules.

As @kba pointed out, there's actually a strong dependency between the resource_manager-module to the time, when the constants XDG_CONFIG_HOME and XDG_DATA_HOME are loaded / evaluated.
The trouble with testing the resource_manager originates from the - somehow intransparent - relationship to the execution environment i.e. context - intransparent to the caller in the moment of execution, which makes it difficult to test any depending functionalities, since testing is a rather artificial execution. The rather plain "workaround" for this bias is the possibility to pass these this info to the objects which require them, as properties or at initialization time. I found myself often ending up in implementations like this, which only exist to ease testing purposes and nothing else.

This way there's no need to run the three resource_manager tests separate from regular unit tests.

Personally, I tried even to go further this line, which simply results in having no "constant" values for HOME or the xdg-stuff, but rather evaluated both only when they are really required, but this interferes with other modules - os.py and bashlib.py, the latter which I do not yet understand.