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.

src/testdir/dumps/Test_syntax_c_01.dump

@@ -1,20 +1,20 @@
-|/+0#0000e05#ffffff16|*| |c|o|m@1|e|n|t| |l|i|n|e| |a|t| |t|h|e| |t|o|p| |*|/| +0#0000001&@45
-| @1|i+0#00e0003&|n|t| +0#0000001&@69
-|m|a|i|n|(|i+0#00e0003&|n|t| +0#0000001&|a|r|g|c|,| |c+0#00e0003&|h|a|r| +0#0000001&|*@1|a|r|g|v|)|/+0#0000e05&@1| |a|n|o|t|h|e|r| |c|o|m@1|e|n|t| +0#0000001&@29
+|/+0#0000e05#ffffff0|*| |c|o|m@1|e|n|t| |l|i|n|e| |a|t| |t|h|e| |t|o|p| |*|/| +0#0000000&@45
+| @1|i+0#00e0003&|n|t| +0#0000000&@69
+|m|a|i|n|(|i+0#00e0003&|n|t| +0#0000000&|a|r|g|c|,| |c+0#00e0003&|h|a|r| +0#0000000&|*@1|a|r|g|v|)|/+0#0000e05&@1| |a|n|o|t|h|e|r| |c|o|m@1|e|n|t| +0#0000000&@29
 |{| @73
-|#+0#e000e06&|i|f| |0| +0#0000001&@69
-| +0#0000e05&@2|i|n|t| @2|n|o|t|_|u|s|e|d|;| +0#0000001&@56
-|#+0#e000e06&|e|l|s|e| +0#0000001&@69
-| @2|i+0#00e0003&|n|t| +0#0000001&@2|u|s|e|d|;| @60
-|#+0#e000e06&|e|n|d|i|f| +0#0000001&@68
-| @2|p|r|i|n|t|f|(|"+0#e000002&|J|u|s|t| |a|n| |e|x|a|m|p|l|e| |p|i|e|c|e| |o|f| |C| |c|o|d|e|\+0#e000e06&|n|"+0#e000002&|)+0#0000001&|;| @27
-| @2|r+0#af5f00255&|e|t|u|r|n| +0#0000001&|0+0#e000002&|x|0|f@1|;+0#0000001&| @58
+|#+0#e000e06&|i|f| |0| +0#0000000&@69
+| +0#0000e05&@2|i|n|t| @2|n|o|t|_|u|s|e|d|;| +0#0000000&@56
+|#+0#e000e06&|e|l|s|e| +0#0000000&@69
+| @2|i+0#00e0003&|n|t| +0#0000000&@2|u|s|e|d|;| @60
+|#+0#e000e06&|e|n|d|i|f| +0#0000000&@68
+| @2|p|r|i|n|t|f|(|"+0#e000002&|J|u|s|t| |a|n| |e|x|a|m|p|l|e| |p|i|e|c|e| |o|f| |C| |c|o|d|e|\+0#e000e06&|n|"+0#e000002&|)+0#0000000&|;| @27
+| @2|r+0#af5f00255&|e|t|u|r|n| +0#0000000&|0+0#e000002&|x|0|f@1|;+0#0000000&| @58
 |}| @73
-| @2|s+0#00e0003&|t|a|t|i|c| +0#0000001&|v+0#00e0003&|o|i|d| +0#0000001&@60
-|m|y|F|u|n|c|t|i|o|n|(|c+0#00e0003&|o|n|s|t| +0#0000001&|d+0#00e0003&|o|u|b|l|e| +0#0000001&|c|o|u|n|t|,| |s+0#00e0003&|t|r|u|c|t| +0#0000001&|n|o|t|h|i|n|g|,| |l+0#00e0003&|o|n|g| +0#0000001&|t|h|e|r|e|)| |{| @14
-| @1|/+0#0000e05&@1| |1+0#e000002&|2|3|:+0#0000e05&| |n|o|t|h|i|n|g| |t|o| |r|e|a|d| |h|e|r|e| +0#0000001&@44
-| @1|f+0#af5f00255&|o|r| +0#0000001&|(|i+0#00e0003&|n|t| +0#0000001&|i| |=| |0+0#e000002&|;+0#0000001&| |i| |<| |c|o|u|n|t|;| |+@1|i|)| |{| @39
-| @3|b+0#af5f00255&|r|e|a|k|;+0#0000001&| @64
+| @2|s+0#00e0003&|t|a|t|i|c| +0#0000000&|v+0#00e0003&|o|i|d| +0#0000000&@60
+|m|y|F|u|n|c|t|i|o|n|(|c+0#00e0003&|o|n|s|t| +0#0000000&|d+0#00e0003&|o|u|b|l|e| +0#0000000&|c|o|u|n|t|,| |s+0#00e0003&|t|r|u|c|t| +0#0000000&|n|o|t|h|i|n|g|,| |l+0#00e0003&|o|n|g| +0#0000000&|t|h|e|r|e|)| |{| @14
+| @1|/+0#0000e05&@1| |1+0#e000002&|2|3|:+0#0000e05&| |n|o|t|h|i|n|g| |t|o| |r|e|a|d| |h|e|r|e| +0#0000000&@44
+| @1|f+0#af5f00255&|o|r| +0#0000000&|(|i+0#00e0003&|n|t| +0#0000000&|i| |=| |0+0#e000002&|;+0#0000000&| |i| |<| |c|o|u|n|t|;| |+@1|i|)| |{| @39
+| @3|b+0#af5f00255&|r|e|a|k|;+0#0000000&| @64
 | @1|}| @71
 |}| @73
 |"|X|t|e|s|t|.|c|"| |1|9|L|,| |3|6|4|C| @37|1|,|1| @10|A|l@1| 

src/testdir/screendump.vim

@@ -19,9 +19,9 @@ func RunVimInTerminal(arguments, options)
   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)

src/testdir/test_syntax.vim

@@ -428,6 +428,8 @@ func Test_bg_detection()
   set bg=dark
   hi Normal ctermbg=12
   call assert_equal('dark', &bg)
+
+  hi Normal ctermbg=NONE
 endfunc
 
 func Test_syntax_hangs()

src/version.c

@@ -778,6 +778,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1535,
 /**/
     1534,
 /**/