patch 8.0.0384: timer test failed for no apparent reason

Problem:    Timer test failed for no apparent reason.
Solution:   Mark the test as flaky.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 0c0a197..71028a1 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -165,6 +165,7 @@
       \ 'Test_collapse_buffers()',
       \ 'Test_communicate()',
       \ 'Test_nb_basic()',
+      \ 'Test_oneshot()',
       \ 'Test_pipe_through_sort_all()',
       \ 'Test_pipe_through_sort_some()',
       \ 'Test_reltime()',