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/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 941cbd3..cdf8756 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -84,6 +84,7 @@
 	    test_cmdline.res \
 	    test_command_count.res \
 	    test_comparators.res \
+	    test_conceal.res \
 	    test_crypt.res \
 	    test_cscope.res \
 	    test_curswant.res \