tmt attempts to install essential requirements of disabled checks

From https://github.com/teemtee/tmt/blob/main/tmt/steps/prepare/__init__.py:

```python
                for check in test.check:
                    collected_requires[guest].dependencies += check.plugin.essential_requires(
                        guest, test, self._logger
                    )
```

As you can see, even disabled checks contribute to collected requirements.