patch 8.2.3346: Vim9: no error for using "." for concatenation after ":vim9cmd"

Problem:    Vim9: no error for using "." for concatenation after ":vim9cmd".
            (Naohiro Ono)
Solution:   Check for Vim9 script syntax. (closes #8756)

src/testdir/test_vim9_cmd.vim

@@ -14,6 +14,7 @@ def Test_vim9cmd()
   END
   CheckScriptSuccess(lines)
   assert_fails('vim9cmd', 'E1164:')
+  assert_fails('vim9cmd echo "con" . "cat"', 'E15:')
 
   lines =<< trim END
       vim9script