run system tests against enterprise databases

In https://github.com/googleapis/python-firestore/pull/1047, we added a verify_pipeline step to the system tests, allowing us to run queries using the RunQuery rpc, and a Pipelines query, and compare results.

But RunQuery calls are not supported in the enterprise mode private preview, so enterprise databases aren't used for tests currently. When enterprise databases are GA, it should be added to the [TEST_DATABASES list](https://github.com/googleapis/python-firestore/blob/b2546f783b94d2cf20e46bd4ff044edd1d27aacf/tests/system/test__helpers.py#L22) (possibly replacing FIRESTORE_NAMED_DB, if they are interchangeable)