Bug 1617369 - Reformat a rust test using rustfmt r=marco

Depends on D65902

Differential Revision: https://phabricator.services.mozilla.com/D65903

--HG--
extra : moz-landing-system : lando

dom/media/gtest/hello.rs

@@ -1,5 +1,5 @@
 #[no_mangle]
-pub extern fn test_rust() -> *const u8 {
+pub extern "C" fn test_rust() -> *const u8 {
     // NB: rust &str aren't null terminated.
     let greeting = "hello from rust.\0";
     greeting.as_ptr()