tests: add test for cast as selector fix (see also #20717) (#20722)

vlib/v/tests/as_cast_selector_test.v

@@ -0,0 +1,18 @@
+struct Foo {
+mut:
+	x int
+}
+
+struct Bar {
+mut:
+	y int
+}
+
+type Foobar = Bar | Foo
+
+fn test_main() {
+	mut bar := Foobar(Bar{
+		y: 123
+	})
+	assert bar as Bar.y == 123
+}