[test] Split snapshots into separate files + unify update behavior

[wkozyra95]

• Removes a lot of unnecessary error handling and abstractions.
• Old implementation was allocated everything at the beginning and kept it in memory, which was expensive.
• Setup was very complicated to modify (flow was more complicated than necessary) e.g. if you want to change the output format
• Unify update snapshot logic with how integration_tests crates handles it (--feature update_snapshots)

I was also considering:

• moving tests to integration_tests crate
• moving scene.json files next to the test *.rs files
  I think it would be better, but I'm not sure if it's worth making such large changes (in terms of files), just to move it.