patch 9.1.0789: tests: ':resize + 5' has invalid space after '+'

Problem:  tests: ':resize + 5' has invalid space after '+'
Solution: Correct the test (Milly)

closes: #15884

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_window_cmd.vim

@@ -557,7 +557,7 @@ func Test_equalalways_on_close()
   1wincmd w
   split
   4wincmd w
-  resize + 5
+  resize +5
   " left column has three windows, equalized heights.
   " right column has two windows, top one a bit higher
   let height_1 = winheight(1)

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    789,
 /**/
     788,
 /**/