patch 9.1.0915: GVim: default font size a bit too small

Problem:  GVim: default font size a bit too small
Solution: increase guifont size to 12 pt on GTK builds
          of gVim (matveyt).

fixes: #16172
closes: #16178

Signed-off-by: matveyt <matthewtarasov@yandex.ru>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index ded3197..6fc0dcc 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -55,6 +55,10 @@
 
 " In the GUI we can always change the screen size.
 if has('gui_running')
+  if has('gui_gtk')
+    " to keep screendump size unchanged
+    set guifont=Monospace\ 10
+  endif
   set columns=80 lines=25
 endif