patch 8.1.0721: conceal mode is not sufficiently tested

Problem:    Conceal mode is not sufficiently tested.
Solution:   Add screendump tests.  Check all 'concealcursor' values.
diff --git a/src/Make_all.mak b/src/Make_all.mak
index 949d0a3..7b85811 100644
--- a/src/Make_all.mak
+++ b/src/Make_all.mak
@@ -33,6 +33,7 @@
 	test_command_count \
 	test_comparators \
 	test_compiler \
+	test_conceal \
 	test_crypt \
 	test_cscope \
 	test_cursor_func \