add test for #116796

Author: bvanjoi

tests/ui/consts/issue-116796.rs

@@ -0,0 +1,3 @@
+struct X<const FN: fn() = { || {} }>;
+//~^ ERROR using function pointers as const generic parameters is forbidden
+fn main() {}

tests/ui/consts/issue-116796.stderr

@@ -0,0 +1,10 @@
+error: using function pointers as const generic parameters is forbidden
+  --> $DIR/issue-116796.rs:1:20
+   |
+LL | struct X<const FN: fn() = { || {} }>;
+   |                    ^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+
+error: aborting due to 1 previous error
+