Add unit tests for PluginRequirements

[eileen-kuehn]

The current implementation of PluginRequirements does not come with any unit tests yet. Those should be added.

[maxfischer2781]

I'm a bit unsure how to best test this, so looking for input. It's important and fragile enough that coverage is desirable.

The problem is that plugins are handled via entry points, which AFAIK strictly require installing. We cannot really monkey-patch them, as we do for other resources.

Currently, I see two possible approaches:

• Install some "test plugins" as part of cobald itself. We can probably name-mangle them so no-one accidentally uses them, but they would still be there.
  • Alternatively, have a dedicated test package containing the plugins.
• Modify the plugin loading to separate reading and processing. This is the most unittest'ish approach, but complicates plugin loading.