Rename G_TEST_DISTED to G_TEST_DIST

Since this feature is so utterly automake-centric, we may as well be
using the same terminology as automake itself (ie: although it's
BUILT_SOURCES, it's DIST_EXTRA, not DISTED).

Also add some comments to the enum explaining that these terms are
really corresponding directly to the automake terms.

https://bugzilla.gnome.org/show_bug.cgi?id=549783