Use insta for API tests

[Turbo87]

see https://github.com/mitsuhiko/insta

This should hopefully make it easier to write more integration tests for our API in the future :)

r? @jtgeibel

[bors]

☔ The latest upstream changes (presumably #2961) made this pull request unmergeable. Please resolve the merge conflicts.

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[bors]

☔ The latest upstream changes (presumably #2993) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author