patch 8.0.0447: getting font name does not work on X11

Problem:    Getting font name does not work on X11.
Solution:   Implement gui_mch_get_fontname() for X11.  Add more GUI tests.
            (Kazunobu Kuriyama)

src/testdir/test_gui_init.vim

@@ -36,3 +36,25 @@ func Test_set_guiheadroom()
     throw skipped
   endif
 endfunc
+
+func Test_set_guioptions_for_M()
+  sleep 200ms
+  " Check if the 'M' option is included.
+  call assert_match('.*M.*', &guioptions)
+endfunc
+
+func Test_set_guioptions_for_p()
+  let skipped = ''
+
+  if !g:x11_based_gui
+    let skipped = g:not_supported . '''p'' of guioptions'
+  else
+    sleep 200ms
+    " Check if the 'p' option is included.
+    call assert_match('.*p.*', &guioptions)
+  endif
+
+  if !empty(skipped)
+    throw skipped
+  endif
+endfunc