Add regression test when modifying events_default (#368)

* Add regression test when modifying events_default

xref matrix-org/gomatrixserverlib#306

* Update tests/csapi/power_levels_test.go

Author: kegsay

tests/csapi/power_levels_test.go

@@ -0,0 +1,42 @@
+package csapi_tests
+
+import (
+	"testing"
+
+	"github.com/matrix-org/complement/internal/b"
+)
+
+// This test ensures that an authorised (PL 100) user is able to modify the users_default value
+// when that value is equal to the value of authorised user.
+// Regression test for https://github.com/matrix-org/gomatrixserverlib/pull/306
+func TestDemotingUsersViaUsersDefault(t *testing.T) {
+	deployment := Deploy(t, b.BlueprintAlice)
+	defer deployment.Destroy(t)
+
+	alice := deployment.Client(t, "hs1", "@alice:hs1")
+
+	roomID := alice.CreateRoom(t, map[string]interface{}{
+		"preset": "public_chat",
+		"power_level_content_override": map[string]interface{}{
+			"users_default": 100, // the default is 0
+			"users": map[string]interface{}{
+				alice.UserID: 100,
+			},
+			"events":        map[string]int64{},
+			"notifications": map[string]int64{},
+		},
+	})
+
+	alice.SendEventSynced(t, roomID, b.Event{
+		Type:     "m.room.power_levels",
+		StateKey: b.Ptr(""),
+		Content: map[string]interface{}{
+			"users_default": 40, // we change the default to 40. We should be able to do this.
+			"users": map[string]interface{}{
+				alice.UserID: 100,
+			},
+			"events":        map[string]int64{},
+			"notifications": map[string]int64{},
+		},
+	})
+}