go/zodb: persistent tests: Print all problems about checked object, not only the first one

Seeing all problems helps to understand what is going on, compared to
e.g. if it complains only about .jar being wrong.

go/zodb/persistent_test.go

@@ -74,10 +74,9 @@ func checkObj(t testing.TB, obj IPersistent, jar *Connection, oid Oid, serial Ti
 	xbase := reflect.ValueOf(obj).Elem().FieldByName("Persistent")
 	pbase := xbase.Addr().Interface().(*Persistent)
 
+	var badv []string
 	badf := func(format string, argv ...interface{}) {
-		t.Helper()
-		msg := fmt.Sprintf(format, argv...)
-		t.Fatalf("%#v: %s", obj, msg)
+		badv = append(badv, fmt.Sprintf(format, argv...))
 	}
 
 	zc := pbase.zclass
@@ -105,6 +104,14 @@ func checkObj(t testing.TB, obj IPersistent, jar *Connection, oid Oid, serial Ti
 	if pbase.instance != obj {
 		badf("base.instance != obj")
 	}
+
+	if len(badv) != 0 {
+		msg := fmt.Sprintf("%#v:\n", obj)
+		for _, bad := range badv {
+			msg += fmt.Sprintf("\t- %s\n", bad)
+		}
+		t.Fatal(msg)
+	}
 }
 
 // tCheckObj binds checkObj to t.