Hack up the test harness to print what we care about alternate.

Author: khuey

src/tools/compiletest/src/runtest.rs

@@ -589,7 +589,7 @@ impl<'test> TestCx<'test> {
 
         if missing_patterns.len() == 1 {
             self.fatal_proc_rec(
-                &format!("error pattern '{}' not found!", missing_patterns[0]),
+                &format!("error pattern '{}' not found in {output_to_check}!", missing_patterns[0]),
                 proc_res,
             );
         } else {