Update actions/checkout action to v4

Merge pull request #10 from oapi-codegen/chore/testutil

Remove `testutil` dependency

Revert "Merge pull request #9 from oapi-codegen/chore/testutil"

This reverts commit 260c88b, reversing
changes made to 5f8df88.

Merge pull request #9 from oapi-codegen/chore/testutil

Remove `testutil` dependency

Merge pull request #7 from oapi-codegen/chore/iris

Bump Iris

Add gitignore