patch 9.0.0767: 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_tabline.vim b/src/testdir/test_tabline.vim
index 556b859..ab0268d 100644
--- a/src/testdir/test_tabline.vim
+++ b/src/testdir/test_tabline.vim
@@ -154,10 +154,8 @@
       set mouse=a
       exe "norm \<LeftMouse>"
   END
-  call writefile(lines, 'Xclickscript')
+  call writefile(lines, 'Xclickscript', 'D')
   call RunVim([], [], "-e -s -S Xclickscript -c qa")
-
-  call delete('Xclickscript')
 endfunc