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

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>