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/Make_all.mak b/src/testdir/Make_all.mak
index 131ba01..80cf706 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -227,6 +227,7 @@
 	test_searchpos \
 	test_set \
 	test_sha256 \
+	test_shift \
 	test_shortpathname \
 	test_signals \
 	test_signs \