- 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.
Ensuring that the tests work from the build tree requires a growing
number of nasty hacks. The tests also don't verify that the installed
Nix actually works. Thus, the tests now require "make install" to
have been run.
