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/Make_all.mak b/src/Make_all.mak
index 9831f7b..49ad4f0 100644
--- a/src/Make_all.mak
+++ b/src/Make_all.mak
@@ -12,6 +12,7 @@
 	test_autocmd \
 	test_autoload \
 	test_backspace_opt \
+	test_backup \
 	test_blockedit \
 	test_breakindent \
 	test_bufline \