patch 8.0.1640: Test_cwd() is flaky

Problem:    Test_cwd() is flaky.
Solution:   Add to list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index bbcec3a..3a32f45 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -248,6 +248,7 @@
       \ 'Test_close_and_exit_cb()',
       \ 'Test_collapse_buffers()',
       \ 'Test_communicate()',
+      \ 'Test_cwd()',
       \ 'Test_exit_callback_interval()',
       \ 'Test_nb_basic()',
       \ 'Test_oneshot()',