patch 8.2.1077: no enough test coverage for highlighting

Problem:    No enough test coverage for highlighting.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #6351)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 4dca8ea..2a3112a 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -191,6 +191,10 @@
   call assert_equal('"hi default', getreg(':'))
   call feedkeys(":hi c\<S-Tab>\<Home>\"\<CR>", 'xt')
   call assert_equal('"hi clear', getreg(':'))
+  call feedkeys(":hi clear Aardig Aard\<Tab>\<C-B>\"\<CR>", 'xt')
+  call assert_equal('"hi clear Aardig Aardig', getreg(':'))
+  call feedkeys(":hi Aardig \<Tab>\<C-B>\"\<CR>", 'xt')
+  call assert_equal("\"hi Aardig \t", getreg(':'))
 
   " A cleared group does not show up in completions.
   hi Anders ctermfg=green
@@ -201,6 +205,14 @@
   call assert_equal([], getcompletion('A', 'highlight'))
 endfunc
 
+" Test for command-line expansion of "hi Ni " (easter egg)
+func Test_highlight_easter_egg()
+  call test_override('ui_delay', 1)
+  call feedkeys(":hi Ni \<Tab>\<C-B>\"\<CR>", 'xt')
+  call assert_equal("\"hi Ni \<Tab>", @:)
+  call test_override('ALL', 0)
+endfunc
+
 func Test_getcompletion()
   if !has('cmdline_compl')
     return