Fail the build if clippy tests don't pass

This was removed in
de69d556eb (diff-8479eab02701e686aedb15b567dc8fc31220c6e4efb9565ccc9d662b7fee2214)
which caused CI to ignore clippy failures. This adds back the exit,
which should cause CI to fail again if a test is broken (like right
now, as clippy tests are broken on master).

src/bootstrap/test.rs

@@ -799,6 +799,10 @@ impl Step for Clippy {
             // The tests succeeded; nothing to do.
             return;
         }
+
+        if !builder.config.cmd.bless() {
+            crate::detail_exit_macro!(1);
+        }
     }
 }