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/recursive.sh

@@ -4,7 +4,7 @@ sed -i 's/experimental-features .*/& recursive-nix/' "$NIX_CONF_DIR"/nix.conf
 restartDaemon
 
 # FIXME
-if [[ $(uname) != Linux ]]; then exit 99; fi
+if [[ $(uname) != Linux ]]; then skipTest "Not running Linux"; fi
 
 clearStore