I have seen some other programs do this, and think it's pretty cool. Means
you can test wherever it's deployed, as well as at build time.
My other reason for doing it is less happy. Cabal's handling of test suites
sucks, requiring duplicated info, and even when that's done, it fails to
preprocess hsc files here. Building it in avoids that and avoids having
to explicitly tell cabal to enable test suites, which would then make it
link the test executable every time, which is unnecessarily slow.
This also has the benefit that now "make fast test" does a max speed build
and tests it.
