patch 9.0.0719: 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_startup_utf8.vim b/src/testdir/test_startup_utf8.vim
index 33f50a9..e8b99e7 100644
--- a/src/testdir/test_startup_utf8.vim
+++ b/src/testdir/test_startup_utf8.vim
@@ -6,7 +6,7 @@
 
 func Test_read_stdin_utf8()
   let linesin = ['テスト', '€ÀÈÌÒÙ']
-  call writefile(linesin, 'Xtestin')
+  call writefile(linesin, 'Xtestin', 'D')
   let before = [
 	\ 'set enc=utf-8',
 	\ 'set fencs=cp932,utf-8',
@@ -26,8 +26,8 @@
   else
     call assert_equal('', 'RunVimPiped failed.')
   endif
+
   call delete('Xtestout')
-  call delete('Xtestin')
 endfunc
 
 func Test_read_fifo_utf8()
@@ -41,7 +41,7 @@
     throw 'Skipped: bash or zsh is required'
   endif
   let linesin = ['テスト', '€ÀÈÌÒÙ']
-  call writefile(linesin, 'Xtestin')
+  call writefile(linesin, 'Xtestin', 'D')
   let before = [
 	\ 'set enc=utf-8',
 	\ 'set fencs=cp932,utf-8',
@@ -56,8 +56,8 @@
   else
     call assert_equal('', 'RunVim failed.')
   endif
+
   call delete('Xtestout')
-  call delete('Xtestin')
 endfunc
 
 func Test_detect_ambiwidth()
@@ -69,14 +69,13 @@
 	\ 'set ambiwidth=double',
 	\ 'call test_option_not_set("ambiwidth")',
 	\ 'redraw',
-	\ ], 'Xscript')
+	\ ], 'Xscript', 'D')
   let buf = RunVimInTerminal('-S Xscript', #{keep_t_u7: 1})
   call TermWait(buf)
   call term_sendkeys(buf, "S\<C-R>=&ambiwidth\<CR>\<Esc>")
   call WaitForAssert({-> assert_match('single', term_getline(buf, 1))})
 
   call StopVimInTerminal(buf)
-  call delete('Xscript')
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab