patch 9.0.0415: on MS-Windows some tests are flaky

Problem:    On MS-Windows some tests are flaky.
Solution:   Add sleeps, disable swapfile, mark test as flaky. (Ken Takata,
            closes #11082)
diff --git a/src/testdir/test_bufline.vim b/src/testdir/test_bufline.vim
index f3502a3..c592877 100644
--- a/src/testdir/test_bufline.vim
+++ b/src/testdir/test_bufline.vim
@@ -83,6 +83,7 @@
   endif
   call writefile(['call setline(1, "Hello")', 'silent w Xtest', 'q!'], 'Xscript', 'D')
   call system(cmd)
+  sleep 50m
   call assert_equal(['Hello'], readfile('Xtest'))
 
   call delete('Xtest')