The big problem with that is that there is a lot to Excel, and that can cover a lot of different things. Typically, you would want to cover things that the candidate would need to do in their daily job tasks. So it should be "tailored" to the job. Sure, you will want to cover some basic things, such as writing formulas, formatting cells, etc. But what about things like Pivot Tables, Charts, importing/exporting data, etc? Some jobs may have a need for things like that and others may not.
Note that if you Google "Free Online Excel Tests", you will find a whole host of things that may or may not be what you are looking for. If nothing else, they may give you some ideas, and help you on your path to developing your own custom test.
Good luck!