patch 8.1.0251: using full path is not supported for 'backupdir'

Problem:    Using a full path is supported for 'directory' but not for
            'backupdir'. (Mikolaj Machowski)
Solution:   Support 'backupdir' as well. (Christian Brabandt, closes #179)
diff --git a/src/testdir/test_alot.vim b/src/testdir/test_alot.vim
index 1465a7a..16a3f7f 100644
--- a/src/testdir/test_alot.vim
+++ b/src/testdir/test_alot.vim
@@ -2,6 +2,7 @@
 " This makes testing go faster, since Vim doesn't need to restart.
 
 source test_assign.vim
+source test_backup.vim
 source test_bufline.vim
 source test_cd.vim
 source test_changedtick.vim