Use check-pass instead of run-pass

We don't actually care about running these programs, only checking the
warnings they generate.

Author: a2aaron

src/test/ui/lint/let_underscore/let_underscore_drop.rs

@@ -1,4 +1,4 @@
-// run-pass
+// check-pass
 // compile-flags: -W let_underscore_drop
 
 struct NontrivialDrop;

src/test/ui/lint/let_underscore/let_underscore_lock.rs

@@ -1,3 +1,4 @@
+// check-fail
 use std::sync::{Arc, Mutex};
 
 fn main() {

src/test/ui/lint/let_underscore/let_underscore_must_use.rs

@@ -1,4 +1,4 @@
-// run-pass
+// check-pass
 // compile-flags: -W let_underscore_must_use
 
 #[must_use]