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/db-migration.sh

@@ -1,13 +1,13 @@
 # Test that we can successfully migrate from an older db schema
 
+source common.sh
+
 # Only run this if we have an older Nix available
 # XXX: This assumes that the `daemon` package is older than the `client` one
 if [[ -z "${NIX_DAEMON_PACKAGE-}" ]]; then
-    exit 99
+    skipTest "not using the Nix daemon"
 fi
 
-source common.sh
-
 killDaemon
 
 # Fill the db using the older Nix