patch 9.0.0769: too many delete() calls in tests

Problem:    Too many delete() calls in tests.
Solution:   Use deferred delete where possible.
diff --git a/src/testdir/test_winbar.vim b/src/testdir/test_winbar.vim
index 7972bc3..b946bcd 100644
--- a/src/testdir/test_winbar.vim
+++ b/src/testdir/test_winbar.vim
@@ -132,13 +132,12 @@
       wincmd j
       wincmd _
   END
-  call writefile(lines, 'XtestWinbarNotVisble')
+  call writefile(lines, 'XtestWinbarNotVisble', 'D')
   let buf = RunVimInTerminal('-S XtestWinbarNotVisble', #{rows: 10})
   call VerifyScreenDump(buf, 'Test_winbar_not_visible', {})
 
   " clean up
   call StopVimInTerminal(buf)
-  call delete('XtestWinbarNotVisble')
 endfunction
 
 func Test_winbar_not_visible_custom_statusline()
@@ -152,13 +151,12 @@
       wincmd j
       wincmd _
   END
-  call writefile(lines, 'XtestWinbarNotVisble')
+  call writefile(lines, 'XtestWinbarNotVisble', 'D')
   let buf = RunVimInTerminal('-S XtestWinbarNotVisble', #{rows: 10})
   call VerifyScreenDump(buf, 'Test_winbar_not_visible_custom_statusline', {})
 
   " clean up
   call StopVimInTerminal(buf)
-  call delete('XtestWinbarNotVisble')
 endfunction
 
 func Test_drag_statusline_with_winbar()