Assets Dir modifies its path if it is set

In my plugin configuration I set
```
windows:
assets_dir: "${PWD}/assets"
```
Then I get:
`AssertionError: Expecting only one image in /home/srak/project/win11/build/assets/windows`
but also
`ls: cannot access '/home/srak/project/win11/build/assets/windows': No such file or directory`

This seems unintuitive, perhaps assets_dir should be the full path to assets?

No files or tests are named. Start by tracing how the plugin configuration reads and resolves assets_dir, then compare the configured absolute path with the path used in the assertion. Done means the documented or implemented path behavior is consistent and the image lookup uses the intended assets directory.

Written by the indexing model from the issue text.