add integration test for Flux in Enzyme

[CarloLucibello]

we can add to Enzyme's test battery something similar to what is done for Lux
https://github.com/EnzymeAD/Enzyme.jl/blob/main/test/integration/Lux/runtests.jl

[gamila-wisam]

Hi @CarloLucibello,

I added a small set of integration tests in Enzyme.jl to check Enzyme’s gradients on a few Flux models (Dense, Chain, and Conv).

here’s the PR: EnzymeAD/Enzyme.jl#2968
the tests are in Enzyme.jl since they’re checking Enzyme’s gradient computations, but they use Flux models as inputs.

Ready for review.