test: hide progress spinners in test runs

Steady-tick spinner threads redraw straight to the real stderr, bypassing
both the harness capture and the per-test log files: 'Scaffolding' lines
from the pkh new tests kept leaking between test results. The scaffold
tests now pass a hidden draw target (the only MultiProgress not created
by the CLI).

src/new/mod.rs

@@ -331,7 +331,12 @@ mod tests {
     ) -> Result<ScaffoldOutcome, Box<dyn Error>> {
         let previous = std::env::current_dir()?;
         std::env::set_current_dir(dir)?;
-        let result = scaffold(opts, &MultiProgress::new());
+        // Hidden draw target: in tests the spinner would redraw from its
+        // steady-tick thread straight to the real stderr
+        let result = scaffold(
+            opts,
+            &MultiProgress::with_draw_target(crate::ui::progress_draw_target()),
+        );
         std::env::set_current_dir(previous)?;
         result
     }