patch 8.0.0395: testing the + register fails with Motif

Problem:    Testing the + register fails with Motif.
Solution:   Also ignore the "failed to create input context" error in the
            second gvim.  Don't use msg() when it would result in a dialog.
diff --git a/src/testdir/setup_gui.vim b/src/testdir/setup_gui.vim
index 88f538e..90ef1f1 100644
--- a/src/testdir/setup_gui.vim
+++ b/src/testdir/setup_gui.vim
@@ -27,3 +27,6 @@
 func GUITearDownCommon()
   call delete('Xhome', 'rf')
 endfunc
+
+" Ignore the "failed to create input context" error.
+call test_ignore_error('E285')