tests: add union_implementing_interface_test.v (#24857)

vlib/v/tests/unions/union_implementing_interface_test.v

@@ -0,0 +1,20 @@
+interface Speaker {
+	speak() string
+}
+
+union MyUnion implements Speaker {
+	vint    int
+	vu32    u32
+	vstring string
+}
+
+fn (u MyUnion) speak() string {
+	return 'hi, u.vint: ${unsafe { u.vint }}'
+}
+
+fn test_union_implementing_interface() {
+	s := Speaker(MyUnion{
+		vint: 123
+	})
+	assert s.speak() == 'hi, u.vint: 123'
+}