patch 8.1.0926: no test for :wnext, :wNext and :wprevious

Problem:    No test for :wnext, :wNext and :wprevious.
Solution:   Add a test. (Dominique Pelle, closes #3963)
diff --git a/src/testdir/test_alot.vim b/src/testdir/test_alot.vim
index 81873af..204796a 100644
--- a/src/testdir/test_alot.vim
+++ b/src/testdir/test_alot.vim
@@ -66,3 +66,4 @@
 source test_unlet.vim
 source test_virtualedit.vim
 source test_window_cmd.vim
+source test_wnext.vim