Add test for invalid mixed cal

test/built-ins/Temporal/PlainYearMonth/prototype/since/mixed-calendar-invalid.js

@@ -0,0 +1,23 @@
+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.since
+description: Mixed calendars throw as invalid
+features: [Temporal]
+---*/
+
+class customCal extends Temporal.Calendar {
+  constructor () {
+    super('iso8601');
+  }
+  
+  toString() {
+    return "I am a secret cal.";
+  }
+}
+
+const ym1 = new Temporal.PlainYearMonth(2000, 1);
+const ym2 = new Temporal.PlainYearMonth(2000, 1, new customCal());
+
+assert.throws(RangeError, () => ym1.since(ym2), 'since throws with different calendars');