patch 8.2.2415: no way to check for the cmdwin feature

Problem:    No way to check for the cmdwin feature, cmdline_hist is now always
            enabled.
Solution:   Add has('cmdwin') support. Skip arglist test on Windows
            temporarily.
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 8a97488..f26b447 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -19,6 +19,8 @@
 endfunc
 
 func Test_window_cmd_cmdwin_with_vsp()
+  CheckFeature cmdwin
+
   let efmt = 'Expected 0 but got %d (in ls=%d, %s window)'
   for v in range(0, 2)
     exec "set ls=" . v