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_buffer.vim b/src/testdir/test_buffer.vim
index 489926d..9387b5f 100644
--- a/src/testdir/test_buffer.vim
+++ b/src/testdir/test_buffer.vim
@@ -375,6 +375,7 @@
 func Test_buffer_scheme()
   CheckMSWindows
 
+  set noswapfile
   set noshellslash
   %bwipe!
   let bufnames = [
@@ -397,6 +398,7 @@
   endfor
 
   set shellslash&
+  set swapfile&
 endfunc
 
 " this was using a NULL pointer after failing to use the pattern