patch 8.0.0490: vertical split makes 'winfixwidth' window smaller

Problem:    Splitting a 'winfixwidth' window vertically makes it one column
            smaller. (Dominique Pelle)
Solution:   Add one to the width for the separator.
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 58d1390..f7d5317 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -294,20 +294,14 @@
   set winfixwidth
   vsplit Xc
   let [ww1, ww2, ww3] = [winwidth(1), winwidth(2), winwidth(3)]
-  " FIXME: commented out: I would expect the width of 2nd window to 
-  " remain 2 but it's actually 1?!
-  "call assert_equal(2, winwidth(2))
+  call assert_equal(2, winwidth(2))
   call assert_inrange(ww3, ww3 + 1, ww1)
   3wincmd >
-  " FIXME: commented out: I would expect the width of 2nd window to 
-  " remain 2 but it's actually 1?!
-  "call assert_equal(2,       winwidth(2))
+  call assert_equal(2,       winwidth(2))
   call assert_equal(ww1 + 3, winwidth(1))
   call assert_equal(ww3 - 3, winwidth(3))
   wincmd =
-  " FIXME: commented out: I would expect the width of 2nd window to 
-  " remain 2 but it's actually 1?!
-  "call assert_equal(2,   winwidth(2))
+  call assert_equal(2,   winwidth(2))
   call assert_equal(ww1, winwidth(1))
   call assert_equal(ww3, winwidth(3))