vim-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.
6acadda8d6

src/nvim/testdir/test_syntax.vim

@@ -466,6 +466,8 @@ func Test_bg_detection()
   set bg=dark
   hi Normal ctermbg=12
   call assert_equal('dark', &bg)
+
+  hi Normal ctermbg=NONE
 endfunc
 
 fun Test_synstack_synIDtrans()