Made gproc_dist_test sequential

The gproc_dist_test module used the 'inparallel' EUnit macro,
basically in order to improve speed. However, this seems to introduce
some indeterminism. The test has been changed to 'inorder'. This
revealed some bugs, some trailing DOWN messages (this probably didn't
affect the 'inparallel' run, since the test cases ran in different
processes).