patch 8.0.0417: test for the clipboard fails sometimes

Problem:    Test for the clipboard fails sometimes.
Solution:   Add it to the flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 71028a1..d750da5 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -168,6 +168,7 @@
       \ 'Test_oneshot()',
       \ 'Test_pipe_through_sort_all()',
       \ 'Test_pipe_through_sort_some()',
+      \ 'Test_quoteplus()',
       \ 'Test_reltime()',
       \ ]