Oracle unit tests skipped in ci

The tests defined in `tests/unit/ibis_oracle/test_init.py` all have the following decorator:

```python
@pytest.mark.skipif(not get_module_under_test(), reason="No Oracle driver")
```

We do not include `oracledb` package in Nox config:

```python
UNIT_PACKAGES = [
"pyfakefs",
"freezegun",
"teradatasql",
"snowflake-connector-python",
"snowflake-sqlalchemy",
]
```

Therefore the tests are skipped:

```bash
$ pytest tests/unit/ibis_oracle/test_init.py
...
tests/unit/ibis_oracle/test_init.py ssssss [100%]

====================================================================================== 6 skipped in 1.93s
```

Start with tests/unit/ibis_oracle/test_init.py and the Nox configuration containing UNIT_PACKAGES. Run pytest tests/unit/ibis_oracle/test_init.py to confirm the current skips, then inspect how get_module_under_test() detects the Oracle driver. Done means the Oracle unit tests run in the configured environment instead of being skipped.

Written by the indexing model from the issue text.