patch 8.2.0866: not enough tests for buffer writing

Problem:    Not enough tests for buffer writing.
Solution:   Add more tests. Use CheckRunVimInTerminal in more places.
            (Yegappan Lakshmanan, closes #6167)

src/testdir/test_syntax.vim

@@ -597,9 +597,7 @@ endfunc
 
 " Check highlighting for a small piece of C code with a screen dump.
 func Test_syntax_c()
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot make screendumps'
-  endif
+  CheckRunVimInTerminal
   call writefile([
 	\ '/* comment line at the top */',
 	\ 'int main(int argc, char **argv) { // another comment',