patch 8.2.0502: Vim9: some code is not tested

Problem:    Vim9: some code is not tested.
Solution:   Add more tests.  Fix uncovered problems.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index fb084d3..b04e283 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -3663,7 +3663,7 @@
 		}
 		if (skip)	// skip "/pat/"
 		{
-		    cmd = skip_regexp(cmd, c, (int)p_magic, NULL);
+		    cmd = skip_regexp(cmd, c, (int)p_magic);
 		    if (*cmd == c)
 			++cmd;
 		}
@@ -6123,7 +6123,7 @@
     {
 	// ":open /pattern/": put cursor in column found with pattern
 	++eap->arg;
-	p = skip_regexp(eap->arg, '/', p_magic, NULL);
+	p = skip_regexp(eap->arg, '/', p_magic);
 	*p = NUL;
 	regmatch.regprog = vim_regcomp(eap->arg, p_magic ? RE_MAGIC : 0);
 	if (regmatch.regprog != NULL)
@@ -7857,7 +7857,7 @@
     {
 	whole = FALSE;
 	++eap->arg;
-	p = skip_regexp(eap->arg, '/', p_magic, NULL);
+	p = skip_regexp(eap->arg, '/', p_magic);
 	if (*p)
 	{
 	    *p++ = NUL;