add todo test for feature gate

tests/ui/feature-gates/feature-gate-global-registration.rs

@@ -1 +1,3 @@
-//! WIP
+todo!(); //~ ERROR
+
+fn main() {}

tests/ui/feature-gates/feature-gate-global-registration.stderr

@@ -0,0 +1,13 @@
+error: expected one of `!` or `::`, found `(`
+  --> $DIR/feature-gate-global-registration.rs:1:1
+   |
+LL | todo!();
+   | ^^^^^^^
+   | |
+   | expected one of `!` or `::`
+   | in this macro invocation
+   |
+   = note: this error originates in the macro `todo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+