Fixing problem for silently supporting jsonnet.

[muthissar]

What does this PR do?

Fixes problem causing LigthningCLI to not support jsonnet config files. Solution, should not change generality.

Fixes #20898

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📚 Documentation preview 📚: https://pytorch-lightning--20899.org.readthedocs.build/en/20899/

[muthissar]

The pull request fails the test due to jsonnet not being installed in the test environment. I am not sure how this should be handled. Is jsonnet supposed to be supported by LightningCLI? In my experience, it is actually supported and therefore jsonnet could simply be added to test dependencies. However, this might open other issues if users experience other problems. I propose:

1. Removing the test
2. Add jsonnet to test dependencies

[Borda]

2. Add jsonnet to test dependencies

lte's add it to https://github.com/Lightning-AI/pytorch-lightning/blob/master/requirements/pytorch/extra.txt