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/proto/charset.pro b/src/proto/charset.pro
index 2f64077..54af280 100644
--- a/src/proto/charset.pro
+++ b/src/proto/charset.pro
@@ -1,6 +1,7 @@
 /* charset.c */
 int init_chartab(void);
 int buf_init_chartab(buf_T *buf, int global);
+int check_isopt(char_u *isopt);
 void trans_characters(char_u *buf, int bufsize);
 char_u *transstr(char_u *s);
 char_u *str_foldcase(char_u *str, int orglen, char_u *buf, int buflen);