Remove --forked from pytest as workaround for jax 0.4.25

[traversaro]

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📚 Documentation preview 📚: https://jaxsim--94.org.readthedocs.build//94/

[flferretti]

Thanks a lot @traversaro, LGTM

[diegoferigo]

For the records, the --forked was necessary in the past otherwise jax was failing due to multiple jitted functions running of different pytrees with same structure. I never investigated the real reason of failure, and found a quick workaround by running all tests on different processes.

If now CI works without it, I agree on removing --forked.

[traversaro]

Actually the problem was different here, I just forgot to close the PR. With jax 0.4.25 --forked was making the test fail, but since we are not using 0.4.25 we could also leave it there.

[diegoferigo]

If --forked is no longer necessary also for jax<0.4.25, I'd prefer to keep the tests without it. It makes the tests run slower.