layout/tests: Mark operations_dont_panic as slow

It's by far the slowest test (disabling it cuts cargo test time by 2x), and it
hadn't caught much in the past months.

src/layout/tests.rs

@@ -1571,6 +1571,11 @@ fn check_ops_with_options(options: Options, ops: &[Op]) -> Layout<TestWindow> {
 
 #[test]
 fn operations_dont_panic() {
+    if std::env::var_os("RUN_SLOW_TESTS").is_none() {
+        eprintln!("ignoring slow test");
+        return;
+    }
+
     let every_op = [
         Op::AddOutput(0),
         Op::AddOutput(1),