mbe: Add a test confirming that a macro attribute can apply itself recursively

This allows a macro attribute to implement default arguments by
reapplying itself with the defaults filled in, for instance.

Author: joshtriplett

tests/ui/macros/macro-rules-attr-nested.rs

@@ -0,0 +1,24 @@
+//@ run-pass
+//@ check-run-results
+#![feature(macro_attr)]
+
+macro_rules! nest {
+    attr() { struct $name:ident; } => {
+        println!("nest");
+        #[nest(1)]
+        struct $name;
+    };
+    attr(1) { struct $name:ident; } => {
+        println!("nest(1)");
+        #[nest(2)]
+        struct $name;
+    };
+    attr(2) { struct $name:ident; } => {
+        println!("nest(2)");
+    };
+}
+
+fn main() {
+    #[nest]
+    struct S;
+}

tests/ui/macros/macro-rules-attr-nested.run.stdout

@@ -0,0 +1,3 @@
+nest
+nest(1)
+nest(2)