test(backup): Ensure coverage of all exportable models

The basic idea here is to include a test that finds all non-abstract
descendants of our `BaseModel`, filter down to just the exportable ones
(ie, those that set `__include_in_export__ = True`), and then ensure
that each of them is included in at least one of the tests seen in
`test_models.py`. This is done by introducing a new `mark` wrapper class
which ingests all of the "target" models used by the `@targets` test
decorator at init time, allowing us to create an exhaustive list of all
types passed to `@targets`.

Issue: getsentry/team-ospo#156

tests/sentry/backup/test_coverage.py

@@ -0,0 +1,49 @@
+from __future__ import annotations
+
+from typing import Type
+
+from sentry.db.models import BaseModel
+from tests.sentry.backup.test_models import TESTED_MODELS
+
+
+def get_final_derivations_of(model: Type):
+    """A "final" derivation of the given `model` base class is any non-abstract class for the
+    "sentry" app with `BaseModel` as an ancestor. Top-level calls to this class should pass in `BaseModel` as the argument."""
+    out = set()
+    for sub in model.__subclasses__():
+        subs = sub.__subclasses__()
+        if subs:
+            out.update(get_final_derivations_of(sub))
+        if not sub._meta.abstract and sub._meta.db_table and sub._meta.app_label == "sentry":
+            out.add(sub)
+    return out
+
+
+def get_exportable_final_derivations_of(model: Type):
+    """Like `get_final_derivations_of`, except that it further filters the results to include only `__include_in_export__ = True`."""
+    return set(
+        filter(
+            lambda c: getattr(c, "__include_in_export__") is True,
+            get_final_derivations_of(model),
+        )
+    )
+
+
+ALL_EXPORTABLE_MODELS = {c.__name__ for c in get_exportable_final_derivations_of(BaseModel)}
+
+
+# Note: this gets checked at runtime, but better to avoid possible runtime errors and catch it early
+# in CI.
+def test_all_final_derivations_of_django_model_set_included_in_export():
+    missing = set(
+        filter(
+            lambda c: not hasattr(c, "__include_in_export__"),
+            get_final_derivations_of(BaseModel),
+        )
+    )
+    assert not missing
+
+
+def test_exportable_final_derivations_of_django_model_are_tested():
+    untested = ALL_EXPORTABLE_MODELS - TESTED_MODELS
+    assert not untested