test: add test for #61432.

Author: eddyb

src/test/ui/const-generics/issue-61432.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+fn promote<const N: i32>() {
+    // works:
+    //
+    // let n = N;
+    // &n;
+
+    &N;
+}
+
+fn main() {
+    promote::<0>();
+}

src/test/ui/const-generics/issue-61432.stderr

@@ -0,0 +1,8 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/issue-61432.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+