[WIP] Add unit tests for Enclaves

[itstauq]

Description

Add unit tests for Enclaves, with the goal of achieving 100% coverage for the following code paths:

• [x] EnclaveService.setup_enclave
• [ ] EnclaveService.upload_assets
• [ ] EnclaveService.execute_code
• [ ] DatasiteEnclaveService.add
• [ ] DatasiteEnclaveService.get_all
• [ ] DatasiteEnclaveService.request_enclave
• [ ] DatasiteEnclaveService.request_asset_upload
• [ ] DatasiteEnclaveService.request_code_execution
• [ ] enclave_output.py:VerifiableOutput
• [ ] enclave.py:EnclaveInstance

How has this been tested?

• Run pytest and generate coverage report using the following command:

  pytest tests/syft/service/enclave --cov=src/syft/service/enclave --cov-report html -n auto --dist loadgroup --durations=20 --disable-warnings
  

• Access the coverage report by opening packages/syft/htmlcov/index.html in your browser.

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [x] I have labeled this PR with the relevant Type labels
• [x] My changes are covered by tests