Add extra test for params

[christiangnrd]

Adding the test suggested by @mcabbott in pull-request #2048 to prevent future regression.

If there are any more cases that you think should be added, let me know.

I made a separate pull-request because I feel that this test should be added regardless of the stats of #2048.

[mcabbott]

I don't love size.(Flux.params(m)) as I'm not sure the order is guaranteed, but lots of other tests above do the same.

Maybe the comment can be clearer too, otherwise fine, thanks!

[christiangnrd]

I used size.(Flux.params(m)) because all the other tests do the same. I agree about the comment being vague, l wrote it as specific as I could given my current familiarity with the code :)