patch 8.2.2979: not all options code is covered by tests

Problem:    Not all options code is covered by tests.
Solution:   Add more tests for options. (Yegappan Lakshmanan, closes #8369)
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index fda34eb..af7c63a 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -397,7 +397,15 @@
   call assert_inrange(ww1, ww1 + 1, ww2)
   call assert_inrange(ww3, ww3 + 1, ww2)
 
-  bw Xa Xb Xc
+  " when the current window width is less than the new 'winwidth', the current
+  " window width should be increased.
+  enew | only
+  split
+  10vnew
+  set winwidth=15
+  call assert_equal(15, winwidth(0))
+
+  %bw!
 endfunc
 
 func Test_equalalways_on_close()