Add ui test of const fn

tests/ui/const-fn.rs

@@ -0,0 +1,6 @@
+use no_panic::no_panic;
+
+#[no_panic]
+const fn f() {}
+
+fn main() {}

tests/ui/const-fn.stderr

@@ -0,0 +1,9 @@
+error[E0015]: cannot call non-const closure in constant functions
+ --> tests/ui/const-fn.rs:3:1
+  |
+3 | #[no_panic]
+  | ^^^^^^^^^^^
+  |
+  = note: closures need an RFC before allowed to be called in constant functions
+  = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  = note: this error originates in the attribute macro `no_panic` (in Nightly builds, run with -Z macro-backtrace for more info)