add regression test for pre-expansion expect lints

Author: Bryntet

tests/ui/lint/pre-expansion-expect.rs

@@ -0,0 +1,7 @@
+// Regression test #155008
+//@ edition: 2015
+//@ check-pass
+#[expect(keyword_idents_2024)]
+fn main() {
+    let gen = 5;
+}