patch 8.0.0361: GUI initialisation is not sufficiently tested

Problem:    GUI initialisation is not sufficiently tested.
Solution:   Add the gui_init test. (Kazuki Kuriyama)

src/testdir/setup_gui.vim

@@ -0,0 +1,29 @@
+" Common preparations for running GUI tests.
+
+let g:x11_based_gui = has('gui_athena') || has('gui_motif')
+	\ || has('gui_gtk2') || has('gui_gnome') || has('gui_gtk3')
+
+" Reasons for 'skipped'.
+let g:not_supported   = "Skipped: Feature/Option not supported by this GUI: "
+let g:not_implemented = "Skipped: Test not implemented yet for this GUI"
+let g:not_hosted      = "Skipped: Test not hosted by the system/environment"
+
+" For KDE set a font, empty 'guifont' may cause a hang.
+func GUISetUpCommon()
+  if has("gui_kde")
+    set guifont=Courier\ 10\ Pitch/8/-1/5/50/0/0/0/0/0
+  endif
+
+  " Gnome insists on creating $HOME/.gnome2/, set $HOME to avoid changing the
+  " actual home directory.  But avoid triggering fontconfig by setting the
+  " cache directory.  Only needed for Unix.
+  if $XDG_CACHE_HOME == '' && exists('g:tester_HOME')
+    let $XDG_CACHE_HOME = g:tester_HOME . '/.cache'
+  endif
+  call mkdir('Xhome')
+  let $HOME = fnamemodify('Xhome', ':p')
+endfunc
+
+func GUITearDownCommon()
+  call delete('Xhome', 'rf')
+endfunc