patch 8.2.2837: various code lines not covered by tests

Problem:    Various code lines not covered by tests.
Solution:   Add test cases. (Dominique Pellé, closes #8178)

src/testdir/test_syntax.vim

@@ -111,6 +111,9 @@ func Test_syntime()
   let a = execute('syntime report')
   call assert_equal("\nNo Syntax items defined for this buffer", a)
 
+  let a = execute('syntime clear')
+  call assert_equal("\nNo Syntax items defined for this buffer", a)
+
   view ../memfile_test.c
   setfiletype cpp
   redraw