BigQuery: Fix unit tests for new versions of fastparquet and pytest

[tswast]

fastparquet is an optional dependency, and is redundant with pyarrow. It is not necessary to include in in the [all] extras. This avoids an issue where the fastparquet dependency pulls in a pre-release version of numpy.

Closes #8549

Version 5.0.0 of pytest modifies the behavior of str on the pytest.raises() context manager. Update the tests to call str on the actual exception (context.value), instead.

[tswast]

System test failures test_list_datasets and test_list_datasets_w_project appear to be flakes.

[tswast]

There's an ongoing issue on the BigQuery backend for dataset.list. Those are the only test failures.