Improve unit tests for contract#validate

- Rename tests with the same name
- Separate test with many assertions on different tests
- Refactor test that is doing integration tests and add mock/stubs to
  make it fit into a unit test
Remove contract unit test that is outdated
Improve unit tests for contract#instantiate

- Separate test with many assertions on different tests
Style improvement of the tests for contract
Upadte README to include the new way to validate responses