patch 8.2.0270: some code not covered by tests

Problem:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes #5649)
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 24501e1..362b0eb 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -940,6 +940,15 @@
   new
   call assert_fails('only', 'E445:')
   only!
+  " Test for :only with a count
+  let wid = win_getid()
+  new
+  new
+  3only
+  call assert_equal(1, winnr('$'))
+  call assert_equal(wid, win_getid())
+  call assert_fails('close', 'E444:')
+  call assert_fails('%close', 'E16:')
 endfunc
 
 " Test for errors with :wincmd