checker: add tests for assign sum type to non sum type error

vlib/v/checker/tests/assign_sumtype2_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/assign_sumtype2_err.vv:13:3: error: cannot assign to field `decl`: expected `Decl`, not `Stmt`
+   11 |     stmt := Stmt(Decl{})
+   12 |     _ := File{
+   13 |         decl: stmt
+      |         ~~~~~~~~~~
+   14 |     }
+   15 | }

vlib/v/checker/tests/assign_sumtype2_err.vv

@@ -0,0 +1,15 @@
+type Stmt = Decl | Expr
+
+struct Decl {}
+struct Expr {}
+
+struct File {
+	decl Decl
+}
+
+fn main() {
+	stmt := Stmt(Decl{})
+	_ := File{
+		decl: stmt
+	}
+}

vlib/v/checker/tests/assign_sumtype_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/assign_sumtype_err.vv:13:9: error: cannot assign to `decl`: expected `Decl`, not `Stmt`
+   11 |     stmt := Stmt(Decl{})
+   12 |     mut decl := Decl{}
+   13 |     decl = stmt
+      |            ~~~~
+   14 | }

vlib/v/checker/tests/assign_sumtype_err.vv

@@ -0,0 +1,14 @@
+type Stmt = Decl | Expr
+
+struct Decl {}
+struct Expr {}
+
+struct File {
+	decl Decl
+}
+
+fn main() {
+	stmt := Stmt(Decl{})
+	mut decl := Decl{}
+	decl = stmt
+}