patch 8.2.1556: cursorline highlighting always overrules sign highlighting

Problem:    Cursorline highlighting always overrules sign highlighting.
Solution:   Combine the highlighting, use the priority to decide how.
            (closes #6812)
diff --git a/src/testdir/test_signs.vim b/src/testdir/test_signs.vim
index e8508b8..3ee2b49 100644
--- a/src/testdir/test_signs.vim
+++ b/src/testdir/test_signs.vim
@@ -1762,6 +1762,20 @@
   call term_sendkeys(buf, ":sign unplace 10\<CR>")
   call VerifyScreenDump(buf, 'Test_sign_cursor_4', {})
 
+  " 'cursorline' highlighting overrules sign
+  call term_sendkeys(buf, ":sign place 12 line=2 name=s2\<CR>")
+  call term_sendkeys(buf, ":set cursorline\<CR>")
+  call term_sendkeys(buf, ":hi CursorLine ctermbg=Green\<CR>")
+  call term_sendkeys(buf, "2G")
+  call term_sendkeys(buf, ":\<CR>")
+  call VerifyScreenDump(buf, 'Test_sign_cursor_5', {})
+
+  " sign highlighting overrules 'cursorline'
+  call term_sendkeys(buf, ":sign unplace 12\<CR>")
+  call term_sendkeys(buf, ":sign place 13 line=2 priority=100 name=s2\<CR>")
+  call term_sendkeys(buf, ":\<CR>")
+  call VerifyScreenDump(buf, 'Test_sign_cursor_6', {})
+
   " clean up
   call StopVimInTerminal(buf)
   call delete('XtestSigncolumn')