patch 9.1.0978: GUI tests sometimes fail when setting 'scroll' options

Problem:  GUI tests sometimes fail when setting 'scroll' options
Solution: decrease the 'scroll' and 'scrolljump' option value from 20 to
          15, in case the Gui window is not large enough to handle 20.

tests: decrease the scroll and scrolljump values

the gui tests sometimes fail with:

```
From test_options_all.vim:
Found errors in Test_opt_set_scroll():
Caught exception in Test_opt_set_scroll(): Vim(set):E49: Invalid scroll size: scroll=20 @ command line..script /home/runner/work/vim/vim/src/testdir/runtest.vim[617]..function RunTheTest[57]..Test_opt_set_scroll, line 7
Found errors in Test_opt_set_scrolljump():
Caught exception in Test_opt_set_scrolljump(): Vim(set):E49: Invalid scroll size: scrolljump=20 @ command line..script /home/runner/work/vim/vim/src/testdir/runtest.vim[617]..function RunTheTest[57]..Test_opt_set_scrolljump, line 9
```

closes: #16337

Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: chrisbra

src/testdir/gen_opt_test.vim

@@ -88,8 +88,8 @@ let test_values = {
       \ 'numberwidth': [[1, 4, 8, 10, 11, 20], [-1, 0, 21]],
       \ 'regexpengine': [[0, 1, 2], [-1, 3, 999]],
       \ 'report': [[0, 1, 2, 9999], [-1]],
-      \ 'scroll': [[0, 1, 2, 20], [-1, 999]],
-      \ 'scrolljump': [[-100, -1, 0, 1, 2, 20], [-101, 999]],
+      \ 'scroll': [[0, 1, 2, 15], [-1, 999]],
+      \ 'scrolljump': [[-100, -1, 0, 1, 2, 15], [-101, 999]],
       \ 'scrolloff': [[0, 1, 8, 999], [-1]],
       \ 'shiftwidth': [[0, 1, 8, 999], [-1]],
       \ 'sidescroll': [[0, 1, 8, 999], [-1]],

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    978,
 /**/
     977,
 /**/