patch 9.0.0669: 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_restricted.vim b/src/testdir/test_restricted.vim
index f743fbf..d01406e 100644
--- a/src/testdir/test_restricted.vim
+++ b/src/testdir/test_restricted.vim
@@ -101,7 +101,7 @@
     call writefile(v:errors, 'Xresult')
     qa!
   END
-  call writefile(lines, 'Xrestricted')
+  call writefile(lines, 'Xrestricted', 'D')
   if RunVim([], [], '-Z --clean -S Xrestricted')
     call assert_equal([], readfile('Xresult'))
   endif
@@ -114,7 +114,6 @@
     call assert_equal([], readfile('Xresult'))
   endif
 
-  call delete('Xrestricted')
   call delete('Xresult')
 endfunc