patch 9.0.0631: 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_prompt_buffer.vim b/src/testdir/test_prompt_buffer.vim
index 8918bd4..fb4b945 100644
--- a/src/testdir/test_prompt_buffer.vim
+++ b/src/testdir/test_prompt_buffer.vim
@@ -237,7 +237,7 @@
         \ done'], #{out_io: 'buffer', out_name: ''})
     startinsert
   END
-  eval script->writefile(scriptName)
+  eval script->writefile(scriptName, 'D')
 
   let buf = RunVimInTerminal('-S ' .. scriptName, {})
   call WaitForAssert({-> assert_equal('cmd:', term_getline(buf, 1))})
@@ -250,7 +250,6 @@
   call WaitForAssert({-> assert_equal('cmd:testtesttest', term_getline(buf, 1))})
 
   call StopVimInTerminal(buf)
-  call delete(scriptName)
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab