CCall testsuite doesn't test `signed` arguments and results

Currently the CCall testsuite fails to exercise the Cmm signed hint. This would have helped catch ghc#20735 (closed).

assigned to @bgamari

mentioned in issue ghc#20735 (closed)

mentioned in commit a4898570

mentioned in merge request !6 (merged)

mentioned in commit 74fc7427

mentioned in commit e699643f

mentioned in commit 7272d293

mentioned in merge request !7 (merged)

mentioned in commit c9db1927

closed with commit 7272d293

mentioned in commit 30adb679