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/version.c b/src/version.c
index 60dadf1..4b216f3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -796,6 +796,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    721,
+/**/
     720,
 /**/
     719,