tests: add test for using reference of map type as struct field (PR #15828) (#15830)

Author: shove70

vlib/v/gen/c/testdata/ref_sumtype_as_struct_field.out

@@ -0,0 +1,4 @@
+Ptr{
+    ptr1: &nil
+    ptr2: &nil
+}

vlib/v/gen/c/testdata/ref_sumtype_as_struct_field.vv

@@ -0,0 +1,15 @@
+struct S1 {}
+
+struct S2 {}
+
+type T1 = S1 | S2
+
+struct Ptr {
+	ptr1 &T1 = unsafe { nil }
+	ptr2 &map[int]int = unsafe { nil }
+}
+
+fn main() {
+	ptr := Ptr{}
+	println(ptr)
+}