patch 8.1.1841: no test for Ex shift commands

Problem:    No test for Ex shift commands.
Solution:   Add a test. (Dominique Pelle, closes #4801)
diff --git a/src/testdir/test_alot.vim b/src/testdir/test_alot.vim
index a7db66a..ab1dac4 100644
--- a/src/testdir/test_alot.vim
+++ b/src/testdir/test_alot.vim
@@ -52,6 +52,7 @@
 source test_scroll_opt.vim
 source test_searchpos.vim
 source test_set.vim
+source test_shift.vim
 source test_sort.vim
 source test_sha256.vim
 source test_statusline.vim