Migrate the unit tests to insta

[andylokandy]

fastrace heavily utilizes snapshot tests, and insta is an appealing snapshot tools to improve the experience of managing the snapshot tests.

[kutal10]

Have taken a look at this, happy to pick up. Have a question though, on top of the crate dependency itself, we have two options here for actually reviewing snapshots:

1. Have every dev do cargo install cargo-insta and use cargo insta review. Much cleaner, but this introduces a manual dependency on the reviewers' local machine.
2. We can go down the environment variable route for Insta, but this is going to need devs to manually review the newly generated snapshots.

Have a read, let me know which route we should go down. I'll be documenting this properly, whichever route we go down.

[andylokandy]

I'll be choosing cargo-insta for my development setup since it provides the complete experience of insta. However it is not required if one want to build the lib or run the tests.