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/test_gui_init.vim

@@ -0,0 +1,35 @@
+" Tests specifically for the GUI features/options that need to be set up at
+" startup to take effect at runtime.
+
+if !has('gui') || ($DISPLAY == "" && !has('gui_running'))
+  finish
+endif
+
+source setup_gui.vim
+
+func Setup()
+  call GUISetUpCommon()
+endfunc
+
+func TearDown()
+  call GUITearDownCommon()
+endfunc
+
+" Make sure that the tests will be done with the GUI activated.
+gui -f
+
+func Test_set_guiheadroom()
+  let skipped = ''
+
+  if !g:x11_based_gui
+    let skipped = g:not_supported . 'guiheadroom'
+  else
+    " The 'expected' value must be consistent with the value specified with
+    " gui_init.vim.
+    call assert_equal(0, &guiheadroom)
+  endif
+
+  if !empty(skipped)
+    throw skipped
+  endif
+endfunc