patch 9.0.0625: 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_lambda.vim b/src/testdir/test_lambda.vim
index 1a69a9b..3e348a0 100644
--- a/src/testdir/test_lambda.vim
+++ b/src/testdir/test_lambda.vim
@@ -354,7 +354,7 @@
       return 1
     endfunc
   END
-  call writefile(l, 'Xscript')
+  call writefile(l, 'Xscript', 'D')
   let caught_932 = 0
   try
     source Xscript
@@ -362,7 +362,6 @@
     let caught_932 = 1
   endtry
   call assert_equal(1, caught_932)
-  call delete('Xscript')
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab