Refactor integration test test cases

[klieret]

Waiting for #290 to be merged.

Currently, test cases are specified as

class TestTrainCase:
    model: str = "graphtcn"
    loss_weights: str = "default"
    ec_params: dict[str, Any] | None = None

and then later there's a long if, elif change turning these strings into the actual classes.

Better would be to directly do

test_case = TestTrainCase(
	model=GraphTCN(
          node_indim,
          edge_indim,
          h_dim=2,
          hidden_dim=2,
          L_ec=2,
          L_hc=2,
      )
)

etc.