vim-patch:8.0.1179: Test_popup_and_window_resize() does not always pass

Problem:    Test_popup_and_window_resize() does not always pass.
Solution:   Do not use $VIMPROG, pass the Vim executable in the vimcmd file.
            (Ozaki Kiichi, closes vim/vim#2186)
631820536e

vim-patch:8.0.1526: no test using a screen dump yet

Problem:    No test using a screen dump yet.
Solution:   Add a test for C syntax highlighting.  Add helper functions.
da65058a9c

NOTE: uses modified `GetVimProg()` (which is used with skipped tests only
(mostly because of `!has('terminal')`)).

Vim uses a 'vimcmd' file, while Nvim uses `$NVIM_TEST_ARGX` environment
variables.

Ref: https://github.com/vim/vim/pull/4806

src/nvim/testdir/test_syntax.vim

@@ -470,7 +470,7 @@ func Test_bg_detection()
   hi Normal ctermbg=NONE
 endfunc
 
-fun Test_synstack_synIDtrans()
+func Test_synstack_synIDtrans()
   new
   setfiletype c
   syntax on
@@ -494,6 +494,39 @@ fun Test_synstack_synIDtrans()
   bw!
 endfunc
 
+" Check highlighting for a small piece of C code with a screen dump.
+func Test_syntax_c()
+  if !has('terminal')
+    return
+  endif
+  call writefile([
+	\ '/* comment line at the top */',
+	\ '  int',
+	\ 'main(int argc, char **argv)// another comment',
+	\ '{',
+	\ '#if 0',
+	\ '   int   not_used;',
+	\ '#else',
+	\ '   int   used;',
+	\ '#endif',
+	\ '   printf("Just an example piece of C code\n");',
+	\ '   return 0x0ff;',
+	\ '}',
+	\ '   static void',
+	\ 'myFunction(const double count, struct nothing, long there) {',
+	\ '  // 123: nothing to read here',
+	\ '  for (int i = 0; i < count; ++i) {',
+	\ '    break;',
+	\ '  }',
+	\ '}',
+	\ ], 'Xtest.c')
+  let buf = RunVimInTerminal('Xtest.c', {})
+  call VerifyScreenDump(buf, 'Test_syntax_c_01')
+  call StopVimInTerminal(buf)
+
+  call delete('Xtest.c')
+endfun
+
 " Using \z() in a region with NFA failing should not crash.
 func Test_syn_wrong_z_one()
   new