tests: general cleanup and fixes for runnning under docker

This now allows tests to be run under a locally built or production
docker image with something like:

  `docker run --rm -v $PWD:/usr/src/paperless --entrypoint=bash paperlessngx/paperless-ngx:latest -c "uv run pytest"`

Specific fixes:
- fix unreachable code around `assertRaises` blocks
- fix `assertInt` typos
- fix `str(e)` vs `str(e.exception)` issues
- skip permission-based checks when root (in a docker container)
- catch `OSError` problems when instantiating `INotify` and
  skip inotify-based tests when it's unavailable.

docs/development.md

@@ -152,6 +152,19 @@ $ ng build --configuration production
     configuration. This is not ideal. But for now, make sure no settings
     except for DEBUG are overridden when testing.
 
+-   Testing under docker is possible (but generally unsupported) if a
+    bare-metal environment cannot be setup. To run tests under docker, use
+    this command:
+
+    ```bash
+    # paperless-ngx/
+
+    $ docker run --rm -i -t \
+      -v $PWD:/usr/src/paperless \
+      --entrypoint=uv paperlessngx/paperless-ngx:latest \
+      run pytest
+    ```
+
 !!! note
 
       The line length rule E501 is generally useful for getting multiple