add more test for -> operator

Author: tyru

test/test_method.vim

@@ -3,3 +3,4 @@ echo 'alice'->s:hello('bob')
 echo [1,2,3]->map({i,n -> n * 2})
 echo 'john'->{s -> 'hello ' .. s }()
 echo 'john'->{s,b -> 'hello ' .. s .. ' ... and goodbye ' .. b .. '!' }('bob')
+echo 'john'->{... -> 'hello ' .. a:000->join(', ') }('bob')