There are no tests for mypy.plugins.proper_plugin

[wyattscarpenter]

There don't seem to be any tests for mypy.plugins.proper_plugin, which was manually tested for its addition in #16036.

Since mypy apparently owns this code, we should test it.

I've figured out this works as a unit test except that we can't import mypy.types in a unit test.

[case properPlugin]
# flags: --config=tmp/foo.ini
from mypy.types import Type, CallableType
result: Type | None = None
isinstance(result, CallableType)                      
[file foo.ini]
\[mypy]
plugins=mypy.plugins.proper_plugin
[builtins fixtures/isinstancelist.pyi]

I've recently realized this plugin parents its notes wrong, so I'd like to include a regression test for that, but I am having a hard time figuring out how to trigger it.