Add binding decoration to string test

This CL adds a test for the binding decoration to_str method.

Bug: tint:11
Change-Id: Id9d16bdb68c4b68fb8bd67dae898f8b7a17ee6c9
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16468
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>

src/ast/binding_decoration_test.cc

@@ -34,5 +34,13 @@ TEST_F(BindingDecorationTest, Is) {
   EXPECT_FALSE(d.IsSet());
 }
 
+TEST_F(BindingDecorationTest, ToStr) {
+  BindingDecoration d{2};
+  std::ostringstream out;
+  d.to_str(out);
+  EXPECT_EQ(out.str(), R"(BindingDecoration{2}
+)");
+}
+
 }  // namespace ast
 }  // namespace tint