Can not use Trainer.test() if train and val dataloaders are not defined
🐛 Bug
When the model does not define train_dataloader and no val_dataloader, we can not use trainer.test(model, test_dataloaders=test_dl).
The configuration checks fail with a MisconfigurationException.
Code sample

model = ...  # a model with no `train_dataloader`, `val_dataloader` defined
test_dl = ... # a dataloader
trainer = pl.Trainer()
trainer.test(model, test_dataloaders=test_dl)

Expected behavior
We expect the testing loop to execute.