Fix the empty-impl tests

Use an existing type so that it compiles.

Author: Jakub

src/test/pretty/empty-impl.pp

@@ -1,5 +1,5 @@
 trait X { }
-impl X for T;
+impl X for uint;
 
 trait Y { }
-impl Y for T;
+impl Y for uint;

src/test/pretty/empty-impl.rs

@@ -1,5 +1,5 @@
 trait X { }
-impl X for T;
+impl X for uint;
 
 trait Y { }
-impl Y for T;
+impl Y for uint;