vim-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 vim/vim#11082)

0500e87eba

Co-authored-by: K.Takata <kentkt@csc.jp>

test/old/testdir/test_buffer.vim

@@ -379,6 +379,7 @@ endfunc
 func Test_buffer_scheme()
   CheckMSWindows
 
+  set noswapfile
   set noshellslash
   %bwipe!
   let bufnames = [
@@ -401,6 +402,7 @@ func Test_buffer_scheme()
   endfor
 
   set shellslash&
+  set swapfile&
 endfunc
 
 " this was using a NULL pointer after failing to use the pattern