Move testing utilities created in `hopr-core`, `hopr-ethereum`, and `hopr-ethereum` into `hopr-testing`

- [x] Move everything that is related to testing and to be used only for `test` or `build` time into `hopr-testing`. Please bear in mind everything included in this repository will not be included into the deliverable.
- [x] Create a new package for `hopr-testing` using our `.github/workflow` from other repositories.