Chore: State the test directory layout in one place (#14171)

The temp directory layout used by the tests was written out four separate
times: once in the documents conftest, once in the paperless checks tests,
once in a fixture local to the NFC upload tests, and once in the helper
behind the old paperless_environment context manager. Each copy covered a
different subset of the settings, so which directories a test actually got
depended on which copy it happened to reach.

The layout now lives in paperless_testing.dirs. build_paperless_dirs owns
where things go and creates them, dirs_settings maps them onto Django
setting names and is pure, and a paperless_dirs fixture in the root conftest
applies that mapping through pytest-django's settings fixture so every app
can reach it. Tests that need a second environment part way through a test
body use the paperless_environment context manager from the same module,
which expresses the identical layout through override_settings. The three
redundant implementations and the old media settings fixture are gone, and
their consumers now take paperless_dirs.

src/documents/tests/utils.py

@@ -79,17 +79,6 @@ def remove_dirs(dirs) -> None:
     dirs.settings_override.disable()
 
 
-@contextmanager
-def paperless_environment():
-    dirs = None
-    try:
-        dirs = setup_directories()
-        yield dirs
-    finally:
-        if dirs:
-            remove_dirs(dirs)
-
-
 def util_call_with_backoff(
     method_or_callable: Callable,
     args: list | tuple,