patch 8.2.0221: no test for Vim9 += and ..=

Problem:    No test for Vim9 += and ..=.
Solution:   Add tests.
diff --git a/src/version.c b/src/version.c
index 74f38b1..f302040 100644
--- a/src/version.c
+++ b/src/version.c
@@ -743,6 +743,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    221,
+/**/
     220,
 /**/
     219,