Add OperatorLearning.jl to Flux downstream tests

[ChrisRackauckas]

This will be a more "Flux intensive" SciML repo than some of the others so I think it will be good to hear early and often about Flux changes there.

CC @pzimbrod

[github-actions]

Once the build has completed, you can preview any updated documentation at this URL: https://fluxml.ai/Flux.jl/previews/PR1869/ in ~20 minutes

[mcabbott]

I'm not sure this is running correctly, but don't see why. https://github.com/FluxML/Flux.jl/actions/runs/1824724204 shows this:

[darsnack]

I don't know why GHA does it this way, but there's a separate "downstream" section that does have the tests. Looks like it ran correctly: https://github.com/FluxML/Flux.jl/actions/runs/1824525398

[mcabbott]

This will need updates for Flux 0.13 BTW, as params is no longer exported:

parameters: Error During Test at /home/runner/work/Flux.jl/Flux.jl/downstream/test/fourierlayer.jl:22
[342](https://github.com/FluxML/Flux.jl/runs/5451760884?check_suite_focus=true#step:6:342)
  Test threw exception
[343](https://github.com/FluxML/Flux.jl/runs/5451760884?check_suite_focus=true#step:6:343)
  Expression: size((params(FourierLayer(128, 64, 100, 20)))[4]) == (1, 64, 100)
[344](https://github.com/FluxML/Flux.jl/runs/5451760884?check_suite_focus=true#step:6:344)
  UndefVarError: params not defined
[345](https://github.com/FluxML/Flux.jl/runs/5451760884?check_suite_focus=true#step:6:345)

(Also, I think 70b4b5a might cure the downstream test weirdness.)