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_preview.vim b/src/testdir/test_preview.vim
index b7b908e..3d13d99 100644
--- a/src/testdir/test_preview.vim
+++ b/src/testdir/test_preview.vim
@@ -40,7 +40,7 @@
   CheckFeature quickfix
 
   filetype on
-  call writefile(['/* some C code */'], 'Xpreview.c')
+  call writefile(['/* some C code */'], 'Xpreview.c', 'D')
   help
   pedit Xpreview.c
   wincmd P
@@ -50,7 +50,6 @@
 
   filetype off
   close
-  call delete('Xpreview.c')
 endfunc
 
 func Test_multiple_preview_windows()