pytests should use `trainer.__init__()` not `hparams` where feasible

[hanlint]

Tests are good code snippets for how to use our trainer. However, most of our tests of trainer functions (e.g. checkpointing, DDP) all rely on the hparams system, which makes it difficult for external users who are mostly non-yahp-based to grok our code. We should migrate all the tests to using trainer=Trainer(..) approaches.