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/shell.sh

@@ -10,7 +10,7 @@ nix shell -f shell-hello.nix hello -c hello NixOS | grep 'Hello NixOS'
 nix shell -f shell-hello.nix hello^dev -c hello2 | grep 'Hello2'
 nix shell -f shell-hello.nix 'hello^*' -c hello2 | grep 'Hello2'
 
-if ! canUseSandbox; then exit 99; fi
+requireSandboxSupport
 
 chmod -R u+w $TEST_ROOT/store0 || true
 rm -rf $TEST_ROOT/store0