Add Harness::new_ui, Harness::fit_contents (#5301)

This adds a `Harness::new_ui`, which accepts a Ui closure and shows the
ui in a central panel. One big benefit is that this allows us to add a
fit_contents method that can run the ui closure with a sizing pass and
resize the "screen" based on the content size.

I also used this to add a snapshot test for the rendering_test at
different scales.

crates/egui_kittest/tests/tests.rs

@@ -0,0 +1,14 @@
+use egui_kittest::Harness;
+
+#[test]
+fn test_shrink() {
+    let mut harness = Harness::new_ui(|ui| {
+        ui.label("Hello, world!");
+        ui.separator();
+        ui.label("This is a test");
+    });
+
+    harness.fit_contents();
+
+    harness.wgpu_snapshot("test_shrink");
+}