add c-variadic miri test

src/tools/miri/tests/pass/c-variadic.rs

@@ -0,0 +1,18 @@
+#![feature(c_variadic)]
+
+use core::ffi::VaList;
+
+fn helper(ap: VaList) -> i32 {
+    // unsafe { ap.arg::<i32>() }
+    let _ = ap;
+    0
+}
+
+unsafe extern "C" fn variadic(a: i32, ap: ...) -> i32 {
+    assert_eq!(a, 42);
+    helper(ap)
+}
+
+fn main() {
+    assert_eq!(unsafe { variadic(42, 1) }, 1);
+}