compiletest: show output in debug logging

I had a test I was confused by; the root issue is that `error-pattern`
runs before normalization, even though `//~ ERROR` runs after
normalization. This logging caught the issue immediately.

src/tools/compiletest/src/runtest/ui.rs

@@ -207,8 +207,12 @@ pub(super) fn run_ui_test(&self) {
 
         debug!(
             "run_ui_test: explicit={:?} config.compare_mode={:?} \
-               proc_res.status={:?} props.error_patterns={:?}",
-            explicit, self.config.compare_mode, proc_res.status, self.props.error_patterns
+               proc_res.status={:?} props.error_patterns={:?} output_to_check={:?}",
+            explicit,
+            self.config.compare_mode,
+            proc_res.status,
+            self.props.error_patterns,
+            output_to_check,
         );
 
         // Compiler diagnostics (expected errors) are always tied to the compile-time ProcRes.