Add extra test to ensure the highlighting for macros is working as expected

tests/rustdoc-html/source-code-pages/macro-call-2.rs

@@ -0,0 +1,18 @@
+// This is yet another test to ensure that only macro calls are considered as such
+// by the rustdoc highlighter, in particular when named `macro_rules`.
+// This is a regression test for <https://github.com/rust-lang/rust/issues/151904>.
+
+#![crate_name = "foo"]
+
+//@ has src/foo/macro-call-2.rs.html
+//@ count - '//code/span[@class="macro"]' 2
+//@ has - '//code/span[@class="macro"]' 'macro_rules!'
+//@ has - '//code/span[@class="macro"]' 'r#macro_rules!'
+
+macro_rules! r#macro_rules {
+    () => {
+        fn main() {}
+    }
+}
+
+r#macro_rules!();