Prevent test discovery from loading models (#915)

Since we started putting models in tests/models, unittest (which
geltest uses also for this) test discovery will go and recursively
import models that are sitting around between test runs. This is rough
as some/all of them are about to be blown away and regenerated.

If we just switched from some branch making interesting changes, the
imports might fail.

unittest has a `load_tests` protocol that we can use to take charge of
our own test loading for a package... and then not do any.

Author: msullivan

gel/_internal/_testbase/_models.py

@@ -358,7 +358,13 @@ async def set_up_class_once(cls, ui: UI) -> None:
         models_dir = test_dir / "models"
         models_dir.mkdir(exist_ok=True)
         # Make sure the base "models" directory has an __init__.py
-        (models_dir / "__init__.py").touch()
+        (models_dir / "__init__.py").write_text(
+            textwrap.dedent('''\
+            # Prevent unittest from recursing into the models
+            def load_tests(*args, **kwargs):
+                return None
+        ''')
+        )
         # and py.typed
         (models_dir / "py.typed").touch()