patch 9.1.0803: tests: no error check when setting global 'isk'

Problem:  tests: no error check when setting global 'isk'
Solution: also parse and check global 'isk' value (Milly)

closes: #15915

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/optiondefs.h b/src/optiondefs.h
index 4a5bd63..42e62e7 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -1456,7 +1456,7 @@
 #endif
 				(char_u *)0L} SCTX_INIT},
     {"iskeyword",   "isk",  P_STRING|P_ALLOCED|P_VIM|P_COMMA|P_NODUP,
-			    (char_u *)&p_isk, PV_ISK, did_set_isopt, NULL,
+			    (char_u *)&p_isk, PV_ISK, did_set_iskeyword, NULL,
 			    {
 				(char_u *)"@,48-57,_",
 #if defined(MSWIN)