Migration towards snapshot-based testing

[bobbbay]

From the cargo-insta documentation,

Snapshots tests (also sometimes called approval tests) are tests that assert values against a reference value (the snapshot). This is similar to how assert_eq! lets you compare a value against a reference value but unlike simple string assertions, snapshot tests let you test against complex values and come with comprehensive tools to review changes.
Snapshot tests are particularly useful if your reference values are very large or change often.

In pest, the reference values are indeed quite large, and change relatively often.

By far, the most popular snapshot-based testing library is cargo-insta, but RA's expect-test also exists.

I do not have the permissions to add labels to this issue, but some labels I would like are good-first-issue and help-wanted.

[CAD97]

Other potentially useful tools include

• trybuild: snapshot style testing for error messages
• macrotest: snapshot style testing for macro expansions