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/fetchGitSubmodules.sh

@@ -2,10 +2,7 @@ source common.sh
 
 set -u
 
-if [[ -z $(type -p git) ]]; then
-    echo "Git not installed; skipping Git submodule tests"
-    exit 99
-fi
+requireGit
 
 clearStore