Improve legacy checkpoint testing #5491
Description
🚀 Feature
Improve legacy checkpoint testing
Motivation
Currently, we only test whether a legacy checkpoint can be loaded and run, but no internal state is checked.
Pitch
Check the epoch number is correct, if the state is the expected, if we achieve a particular loss after 1 step...
We already have similar tests, we just have to do them for legacy checkpoints. see
https://github.com/PyTorchLightning/pytorch-lightning/blob/master/tests/models/test_restore.py
Additional context
cc: @Borda