Re-organize API/integration/implementation tests

I think it would be great if we split out the tests in `microsetta_public_api.api` so the files there are a little less monolithic and more modular.

The module currently has something like the following structure:
```
$ tree -P "test*.py" microsetta_public_api/api
microsetta_public_api/api
├── diversity
│   └── tests
│   └── test_alpha.py
└── tests
├── implementation_tests
│   ├── test_metadata.py
│   └── test_taxonomy.py
├── test_api.py
└── test_integration.py
```

And I think it would be nice to organize it into something more like:
```
microsetta_public_api/api
├── diversity
└── tests
├── api
│ ├── test_alpha.py
│ ├── test_metadata.py
│ └── test_taxonomy.py
├── implementation
│ ├── test_alpha.py
│ ├── test_metadata.py
│ └── test_taxonomy.py
└── integration
├── test_alpha.py
├── test_metadata.py
└── test_taxonomy.py

```

Start by inspecting the existing tests under microsetta_public_api/api/tests, including test_api.py, test_integration.py, and implementation_tests/test_metadata.py and test_taxonomy.py. Compare the current tree with the proposed api, implementation, and integration layout, then run the existing test suite to verify discovery. Done means the tests are organized into the proposed categories and still run successfully.

Written by the indexing model from the issue text.