Add single point energy tests for MACEEMLE

[lohedges]

In relation to #61, we should probably add some single point energy tests for MACEEMLE using the standard MACE-OFF models for the in vacuo backend. This will make sure that our workaround for MACE backwards compatibility is actually working correctly. (It should do for now, but this might not always be the case going forward.)

[JMorado]

I'm happy to add some units test covering this -- I'll make a PR when I've them ready.

[lohedges]

Many thanks. No rush on this. Hopefully there'll be a more robust way of ensuring model compatibility in future.