test: Fix depcrecated alias for int

Author: richo

src/test/run-pass/issue-4759-1.rs

@@ -9,5 +9,5 @@
 // except according to those terms.
 
 trait U { fn f(self); }
-impl U for int { fn f(self) {} }
+impl U for isize { fn f(self) {} }
 pub fn main() { 4.f(); }