add test for already implemented proposal

Closes #1564

test/behavior/sizeof_and_typeof.zig

@@ -292,3 +292,12 @@ test "@sizeOf optional of previously unresolved union" {
     const Node = union { a: usize };
     try expect(@sizeOf(?Node) == @sizeOf(Node) + @alignOf(Node));
 }
+
+test "@offsetOf zero-bit field" {
+    const S = packed struct {
+        a: u32,
+        b: u0,
+        c: u32,
+    };
+    try expect(@offsetOf(S, "b") == @offsetOf(S, "c"));
+}