Convert existential-minimal.rs to compile-pass

Author: jonas-schievink

src/test/run-pass/impl-trait/existential-minimal.stderr

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// run-pass
+// compile-pass
 
 fn main() {}