put cppia output to bin (see #4771)

tests/unit/compile-cppia.hxml

@@ -1,6 +1,6 @@
 compile-each.hxml
 -main unit.Test
--cpp unit.cppia
+-cpp bin/unit.cppia
 -D cppia
 -D NO_PRECOMPILED_HEADERS
 --macro cpp.cppia.HostClasses.exclude()