checker: add test for interface embedding and interface with erroneou…

…s implementation (test related to #21030) (#21033)

vlib/v/checker/tests/struct_init_with_iface_embed_iface_with_incorrect_method_impl_ref_field_err.out

@@ -0,0 +1,9 @@
+vlib/v/checker/tests/struct_init_with_iface_embed_iface_with_incorrect_method_impl_ref_field_err.vv:21:3: error: `&Stream` incorrectly implements method `read` of interface `Refresher`: expected return type `!int`
+   19 |     s := &Stream{}
+   20 |     _ := &Server{
+   21 |         refresher: s
+      |         ~~~~~~~~~~~~
+   22 |     }
+   23 | }
+Details: main.Refresher has `fn read(x main.Refresher, buf []u8) !int`
+         main.Stream has `fn read(a main.Stream, buf []u8)`

vlib/v/checker/tests/struct_init_with_iface_embed_iface_with_incorrect_method_impl_ref_field_err.vv

@@ -0,0 +1,23 @@
+interface Refresher {
+	Reader
+}
+
+interface Reader {
+	read(buf []u8) !int
+}
+
+struct Server {
+	refresher Refresher
+}
+
+struct Stream {}
+
+// Implementation is missing the return type.
+fn (a Stream) read(buf []u8) {}
+
+fn test_struct_init_with_interface_field() {
+	s := &Stream{}
+	_ := &Server{
+		refresher: s
+	}
+}