Make JSON files more human readable (#55335)
Summary:
Prettifies JSON files .pytorch-test-times and .pytorch-slow-tests so that not everything is on one single line.
This is of slightly more importance as generated .pytorch-slow-tests ends up getting stored in our test-infra repo ([example](https://github.com/pytorch/test-infra/commit/ad9cd8756589aa28b6f2817caaf074ca93e76cd9)), and it is nice to not have that lil red symbol at the end.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55335
Reviewed By: samestep
Differential Revision: D27576930
Pulled By: janeyx99
fbshipit-source-id: be58565b8c8593a9bfcfab383ee19facc79f0572
