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/suite.rs

@@ -0,0 +1,32 @@
+//! Every ordinary correctness test, as modules of one target.
+//!
+//! One binary rather than a dozen: each `tests/*.rs` links the whole
+//! dependency graph again, which is most of what `cargo test` spends its time
+//! on here. Libtest still runs the cases in parallel, and a filter still
+//! selects them -- `cargo test --test suite layout::` for one module.
+//!
+//! The rigs stay their own targets: `shrink` and `generated` are fuzzers run
+//! on their own, and the `*_cost` and `*_diagnostics` ones are measurements.
+
+#[path = "cases/determinism.rs"]
+mod determinism;
+#[path = "cases/drift.rs"]
+mod drift;
+#[path = "cases/idempotence.rs"]
+mod idempotence;
+#[path = "cases/layout.rs"]
+mod layout;
+#[path = "cases/pointer.rs"]
+mod pointer;
+#[path = "cases/pointer_routing.rs"]
+mod pointer_routing;
+#[path = "cases/retained.rs"]
+mod retained;
+#[path = "cases/scroll.rs"]
+mod scroll;
+#[path = "cases/tasks.rs"]
+mod tasks;
+#[path = "cases/text_edit.rs"]
+mod text_edit;
+#[path = "cases/unsettled.rs"]
+mod unsettled;