patch 8.2.0480: Vim9: some code is not tested

Problem:    Vim9: some code is not tested.
Solution:   Add more tests.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 584618c..9d66e5e 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -2622,11 +2622,14 @@
 	    else
 	    {
 		// method call:  list->method()
-		for (p = *arg; eval_isnamec1(*p); ++p)
+		p = *arg;
+		if (ASCII_ISALPHA(*p) && p[1] == ':')
+		    p += 2;
+		for ( ; eval_isnamec1(*p); ++p)
 		    ;
 		if (*p != '(')
 		{
-		    semsg(_(e_missing_paren), arg);
+		    semsg(_(e_missing_paren), *arg);
 		    return FAIL;
 		}
 		// TODO: base value may not be the first argument