patch 9.0.0391: using separate delete() call instead of writefile() 'D' flag

Problem:    Using separate delete() call instead of writefile() 'D' flag.
Solution:   Use the writefile 'D' flag.
diff --git a/src/testdir/test_breakindent.vim b/src/testdir/test_breakindent.vim
index 1c02294..f052fb7 100644
--- a/src/testdir/test_breakindent.vim
+++ b/src/testdir/test_breakindent.vim
@@ -902,14 +902,13 @@
       repeat('x', &columns - leftcol - 1)->setline(1)
       'second line'->setline(2)
   END
-  call writefile(lines, 'XscriptShowbreak')
+  call writefile(lines, 'XscriptShowbreak', 'D')
   let buf = RunVimInTerminal('-S XscriptShowbreak', #{rows: 6})
 
   call term_sendkeys(buf, "AX")
   call VerifyScreenDump(buf, 'Test_cursor_position_with_showbreak', {})
 
   call StopVimInTerminal(buf)
-  call delete('XscriptShowbreak')
 endfunc
 
 func Test_no_spurious_match()