patch 8.0.1640: Test_cwd() is flaky

Problem:    Test_cwd() is flaky.
Solution:   Add to list of flaky tests.

src/testdir/runtest.vim

@@ -248,6 +248,7 @@ let s:flaky = [
       \ 'Test_close_and_exit_cb()',
       \ 'Test_collapse_buffers()',
       \ 'Test_communicate()',
+      \ 'Test_cwd()',
       \ 'Test_exit_callback_interval()',
       \ 'Test_nb_basic()',
       \ 'Test_oneshot()',

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1640,
 /**/
     1639,
 /**/