patch 8.0.1535: C syntax test still fails when using gvim

Problem:    C syntax test still fails when using gvim.
Solution:   Clear Normal cterm highlighting instead of setting it.
diff --git a/src/testdir/screendump.vim b/src/testdir/screendump.vim
index 5ebe609..a28cb8c 100644
--- a/src/testdir/screendump.vim
+++ b/src/testdir/screendump.vim
@@ -19,9 +19,9 @@
   split
   vsplit
 
-  " Always doo this with 256 colors and a light background.
-  set t_Co=256
-  hi Normal ctermfg=0 ctermbg=15
+  " Always do this with 256 colors and a light background.
+  set t_Co=256 background=light
+  hi Normal ctermfg=NONE ctermbg=NONE
 
   let cmd = GetVimCommandClean()
   " Add -v to have gvim run in the terminal (if possible)