Introduce E2E Testing Framework

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As sourcify is a free to use, we can add end to end unit tests that will ensure following:

• In case of endpoint response changes, tests will start to fail and we'll need to fix it.
• In case of critical updates to the code, they should not break existing functionality.

We should use assertify suite:
https://pkg.go.dev/github.com/stretchr/testify/suite?utm_source=godoc