patch 8.2.4234: test_garbagecollect_now() does not check v:testing

Problem:    test_garbagecollect_now() does not check v:testing as documented.
Solution:   Give an error if v:testing is not set.
diff --git a/src/version.c b/src/version.c
index bc956ce..7247ca7 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4234,
+/**/
     4233,
 /**/
     4232,