Add a new regression test

This new test highlight the behavior of the new parser and it's ability
to parse variadic rust functions.

gcc/testsuite/ChangeLog:

	* rust/compile/parse_variadic_function.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <[email protected]>

Author: P-E-P

gcc/testsuite/rust/compile/parse_variadic_function.rs

@@ -0,0 +1,4 @@
+fn main() {}
+
+#[cfg(FALSE)]
+fn variadic(x: isize, ...) {}