patch 8.1.1539: not easy to define a variable and lock it

Problem:    Not easy to define a variable and lock it.
Solution:   Add ":const".
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 08f6c5b..9e551c1 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -87,6 +87,7 @@
 	test_comparators \
 	test_compiler \
 	test_conceal \
+	test_const \
 	test_crypt \
 	test_cscope \
 	test_cursor_func \
@@ -312,6 +313,7 @@
 	test_command_count.res \
 	test_comparators.res \
 	test_conceal.res \
+	test_const.res \
 	test_crypt.res \
 	test_cscope.res \
 	test_curswant.res \