build: Don't run the unit tests during distcheck

We're not quite ready to run the unit tests as part of "make distcheck":

 * Some tests fail because they can't find the test data when being run
   from an out-of-tree build, like distcheck does.
 * Failures do not stop the distcheck process, so they quickly scroll
   away, making them hard to spot. And since it takes some time to get
   to the test, you're probably not watching your terminal for several
   minutes...