Check compatibility of `epix_slide` output class with some previous versions

[brookslogan]

Encountered in #290, likely also #259. Seems like merge d8287cf took away the epi_df-producing behavior of d648910, which looks like it was inherited from e957781.

[brookslogan]

Above traceback mightappears to be bogus as the old default group_by behavior might have changed the results for the snippet I was testing.

[brookslogan]

Grouping by geo_value, it looks like we started getting epi_df results in e957781. And still have them in dev. I thought we weren't getting them for grouped or ungrouped archives on lcb/make_epix_slide_more_like_reframe, but apparently that wasn't actually the case. Closing this issue as it appears to have just been a mixup with the testing on that branch + with the old default group_by.