Add SnapshotResults struct to egui_kittest (#5672)

I got annoyed by all the slightly different variations of "collect
snapshot results and unwrap them at the end of test" I've written, so I
added a struct to make this nice and simple.

One controversial thing: It panics when dropped. I wanted to ensure
people cannot forget to unwrap the results at the end, and this was the
best thing I could come up with. I don't think this is possible via
clippy lint or something like that.

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

crates/egui_kittest/tests/tests.rs

@@ -1,4 +1,4 @@
-use egui_kittest::Harness;
+use egui_kittest::{Harness, SnapshotResults};
 
 #[test]
 fn test_shrink() {
@@ -10,6 +10,8 @@ fn test_shrink() {
 
     harness.fit_contents();
 
+    let mut results = SnapshotResults::new();
+
     #[cfg(all(feature = "snapshot", feature = "wgpu"))]
-    harness.snapshot("test_shrink");
+    results.add(harness.try_snapshot("test_shrink"));
 }