Allow selecting "prejoin" events by state keys (#14642)

* Declare new config

* Parse new config

* Read new config

* Don't use trial/our TestCase where it's not needed

Before:

```
$ time trial tests/events/test_utils.py > /dev/null

real	0m2.277s
user	0m2.186s
sys	0m0.083s
```

After:
```
$ time trial tests/events/test_utils.py > /dev/null

real	0m0.566s
user	0m0.508s
sys	0m0.056s
```

* Helper to upsert to event fields

without exceeding size limits.

* Use helper when adding invite/knock state

Now that we allow admins to include events in prejoin room state with
arbitrary state keys, be a good Matrix citizen and ensure they don't
accidentally create an oversized event.

* Changelog

* Move StateFilter tests

should have done this in #14668

* Add extra methods to StateFilter

* Use StateFilter

* Ensure test file enforces typed defs; alphabetise

* Workaround surprising get_current_state_ids

* Whoops, fix mypy

tests/events/test_utils.py

@@ -12,19 +12,20 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import unittest as stdlib_unittest
+
 from synapse.api.constants import EventContentFields
 from synapse.api.room_versions import RoomVersions
 from synapse.events import make_event_from_dict
 from synapse.events.utils import (
     SerializeEventConfig,
     copy_and_fixup_power_levels_contents,
+    maybe_upsert_event_field,
     prune_event,
     serialize_event,
 )
 from synapse.util.frozenutils import freeze
 
-from tests import unittest
-
 
 def MockEvent(**kwargs):
     if "event_id" not in kwargs:
@@ -34,7 +35,31 @@ def MockEvent(**kwargs):
     return make_event_from_dict(kwargs)
 
 
-class PruneEventTestCase(unittest.TestCase):
+class TestMaybeUpsertEventField(stdlib_unittest.TestCase):
+    def test_update_okay(self) -> None:
+        event = make_event_from_dict({"event_id": "$1234"})
+        success = maybe_upsert_event_field(event, event.unsigned, "key", "value")
+        self.assertTrue(success)
+        self.assertEqual(event.unsigned["key"], "value")
+
+    def test_update_not_okay(self) -> None:
+        event = make_event_from_dict({"event_id": "$1234"})
+        LARGE_STRING = "a" * 100_000
+        success = maybe_upsert_event_field(event, event.unsigned, "key", LARGE_STRING)
+        self.assertFalse(success)
+        self.assertNotIn("key", event.unsigned)
+
+    def test_update_not_okay_leaves_original_value(self) -> None:
+        event = make_event_from_dict(
+            {"event_id": "$1234", "unsigned": {"key": "value"}}
+        )
+        LARGE_STRING = "a" * 100_000
+        success = maybe_upsert_event_field(event, event.unsigned, "key", LARGE_STRING)
+        self.assertFalse(success)
+        self.assertEqual(event.unsigned["key"], "value")
+
+
+class PruneEventTestCase(stdlib_unittest.TestCase):
     def run_test(self, evdict, matchdict, **kwargs):
         """
         Asserts that a new event constructed with `evdict` will look like
@@ -391,7 +416,7 @@ class PruneEventTestCase(unittest.TestCase):
         )
 
 
-class SerializeEventTestCase(unittest.TestCase):
+class SerializeEventTestCase(stdlib_unittest.TestCase):
     def serialize(self, ev, fields):
         return serialize_event(
             ev, 1479807801915, config=SerializeEventConfig(only_event_fields=fields)
@@ -513,7 +538,7 @@ class SerializeEventTestCase(unittest.TestCase):
             )
 
 
-class CopyPowerLevelsContentTestCase(unittest.TestCase):
+class CopyPowerLevelsContentTestCase(stdlib_unittest.TestCase):
     def setUp(self) -> None:
         self.test_content = {
             "ban": 50,