Link the ordinary tests once, and keep their debug info to line tables

Eleven `tests/*.rs` were eleven binaries, each linking the whole graph --
`wgpu` and all -- to run a handful of cases. They are modules of one target
now, under `tests/cases/`, and `cargo test --test suite layout::` still picks
one out. The fuzzers and the `*_cost` measurements stay their own targets:
they are run on their own and want to be selectable without building the
rest.

`profile.test` takes `debug = "line-tables-only"`, which is what a backtrace
here actually reads; the type and variable information was the bulk of what
the linker was writing.

Measured on this machine, rebuilding `iris`'s test targets after a change to
the crate: 14.3 s before, 9.8 s with one target, 7.7 s with both. `target/`
went from 45 GB to 13 GB. The suite still passes 102 tests, and the binary
still carries `.debug_line`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/cases/tasks.rs

@@ -0,0 +1,23 @@
+//! What a background task can change, and how it gets back to the ui.
+
+use std::time::Duration;
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+#[test]
+fn a_task_update_reaches_the_tree() {
+    let mut h = Harness::new((400, 200));
+    let widget = rect(Color::RED).add(&mut h.rsc);
+    h.set_root(widget.task_on(CursorSense::click(), async move |mut ctx| {
+        ctx.update(move |_, rsc| widget(rsc).color = Color::BLUE);
+    }));
+
+    h.click((200, 100));
+
+    assert!(
+        h.await_update(Duration::from_secs(5)),
+        "the task sent no update"
+    );
+    assert_eq!(h.rsc[widget].color, Color::BLUE);
+}