Change Harness::run to run until no more repaints are requested (#5580)

Previously, `Harness::run` just called `Harness::step` 3 times. If that
wasn't enough, tests would often call run multiple times so all
animations would finish properly.

Also, I introduced `HarnessBuilder::with_step_dt` to customize with how
big of a dt each frame is called. I set the default to 1.0 / 6.0 (~6fps)
so we don't waste cpu in tests waiting on animations.

`HarnessBuilder::max_steps` allows us to control how many steps
`Harness::run` should run before panicing.
The default is 6, so we run for up to 1.0 logical seconds (six frames at
6 fps), which should be enough to finish most animations.

Turns out a lot of snapshots where rendered before fully shown and had a
light opacity, those are now fixed.

* [x] I have followed the instructions in the PR template

crates/egui_demo_app/tests/test_demo_app.rs

@@ -49,7 +49,7 @@ fn test_demo_app() {
             // Load a local image where we know it exists and loads quickly
             #[cfg(feature = "image_viewer")]
             Anchor::ImageViewer => {
-                harness.run();
+                harness.step();
 
                 harness
                     .get_by_role_and_label(Role::TextInput, "URI:")
@@ -65,7 +65,8 @@ fn test_demo_app() {
             _ => {}
         }
 
-        harness.run();
+        // Can't use Harness::run because fractal clock keeps requesting repaints
+        harness.run_steps(2);
 
         if let Err(e) = harness.try_snapshot(&anchor.to_string()) {
             results.push(e);