chore: remove direct dependency on gotest.tools/v3

[alexandear]

Related Issue or Design Document

The PR replaces usages of gotest.tools/v3 with github.com/stretchr/testify because the latter is mainly used in the repo tests.

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

This refactoring removes gotest.tools/v3 from direct dependencies, which can be seen in the go.mod.