Cleanup test skipping

- Try not to put cryptic "99" in many places

  Factor out `exit 99` into `skipTest` function

- Alows make sure skipping a test is done with a reason

  `skipTest` takes a mandatory argument

- Separate pure conditionals vs side-effectful test skipping.

  "require daemon" already had this, but "sandbox support" did not.

tests/gc-runtime.sh

@@ -4,7 +4,7 @@ case $system in
     *linux*)
         ;;
     *)
-        exit 99;
+        skipTest "Not running Linux";
 esac
 
 set -m # enable job control, needed for kill