Add guidelines about when and how to use snapshot tests effectively

[Gudahtt]

Snapshot tests are often "misused" to create hard-to-understand and brittle tests that are not effective at catching bugs. They can be useful in narrow situations though.

We should write guidance about when and how to effectively use snapshot tests.

There is some mention of snapshot testing here already: https://github.com/MetaMask/contributor-docs/blob/main/docs/testing/unit-testing.md#snapshots
But it doesn't provide enough guidance yet.

References for inspiration:

• https://x.com/searls/status/919594505938112512
• https://www.youtube.com/watch?v=Af4M8GMoxi4