chore(test): add pre batch key condscvi save (#2629)

adds a basic `CondSCVI` save file for backwards compatibility testing in
#2626. generated as follows:
```
adata = synthetic_iid()
CondSCVI.setup_anndata(adata, "labels")
model = CondSCVI(adata)
model.train(max_epochs=1)
model.save(".../tests/test_data/condscvi_pre_batch")
```