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_source_utf8.vim b/src/testdir/test_source_utf8.vim
index d22ce8f..bd7cd8d 100644
--- a/src/testdir/test_source_utf8.vim
+++ b/src/testdir/test_source_utf8.vim
@@ -43,7 +43,7 @@
         \ "map __3 asd\<C-V>\<C-V>",
         \ "map __4 asd\<C-V>\<C-V>\<C-V>",
         \ "map __5 asd\<C-V>\<C-V>\<C-V>",
-        \ ], 'Xtestfile')
+        \ ], 'Xtestfile', 'D')
   source Xtestfile
   enew!
   exe "normal __1\<Esc>\<Esc>__2\<Esc>__3\<Esc>\<Esc>__4\<Esc>__5\<Esc>"
@@ -53,7 +53,6 @@
         \ getline(1, 2))
 
   enew!
-  call delete('Xtestfile')
   unmap __1
   unmap __2
   unmap __3