update test to reflect other sources of brokenness in it under new

macro future proofing rules.

(We may want to think about what this test was actually testing and
figure out a way to test it without running afoul of macro future
proofing.  I spent some time trying to do this, e.g. by inserting
parenthesis in the macro input pattern, but I could not quickly get it
working, so I took this tack instead.)

src/test/compile-fail/issue-30715.rs

@@ -10,7 +10,12 @@
 
 macro_rules! parallel {
     (
-        for $id:ident in $iter:expr {
+        // If future has `pred`/`moelarry` fragments (where "pred" is
+        // "like expr, but with `{` in its FOLLOW set"), then could
+        // use `pred` instead of future-proof erroring here. See also:
+        //
+        // https://github.com/rust-lang/rfcs/pull/1384#issuecomment-160165525
+        for $id:ident in $iter:expr { //~ WARN `$iter:expr` is followed by `{`
             $( $inner:expr; )*
         }
     ) => {};