patch 8.2.3982: some lines of code not covered by tests

Problem:    Some lines of code not covered by tests.
Solution:   Add a few more test cases. (Dominique Pellé, closes #9453)
diff --git a/src/testdir/test_highlight.vim b/src/testdir/test_highlight.vim
index 001cd39..5476008 100644
--- a/src/testdir/test_highlight.vim
+++ b/src/testdir/test_highlight.vim
@@ -922,17 +922,20 @@
   let hlTestHiPre = HighlightArgs('TestHi')
 
   " Test colorscheme
+  call assert_equal("\ndefault", execute('colorscheme'))
   hi clear
   if exists('syntax_on')
     syntax reset
   endif
   let g:colors_name = 'test'
+  call assert_equal("\ntest", execute('colorscheme'))
   hi link TestLink ErrorMsg
   hi TestHi ctermbg=green
 
   " Restore default highlighting
   colorscheme default
   " 'default' should work no matter if highlight group was cleared
+  call assert_equal("\ndefault", execute('colorscheme'))
   hi def link TestLink Identifier
   hi def TestHi ctermbg=red
   let hlTestLinkPost = HighlightArgs('TestLink')