PYTHON-2459 Implement unified GridFS tests

[ShaneHarvey]

https://jira.mongodb.org/browse/PYTHON-2459

This PR implements the unified GridFS tests which is required for CSOT. I've continued skipping the CSOT specific GridFS tests for now since they are failing for other reasons.

[ShaneHarvey]

I'm going to ignore the broken "check links" failures in this PR:

(api/pymongo/cursor: line    3) broken    http://mongodb.com/docs/manual/reference/operator/comment/ - 404 Client Error: Not Found for url: https://www.mongodb.com/docs/manual/reference/operator/meta/comment/
...
(examples/encryption: line  108) broken    https://dochub.mongodb.org/core/client-side-field-level-encryption-automatic-encryption-rules - 404 Client Error: Not Found for url: https://www.mongodb.com/docs/manual/reference/security-client-side-automatic-json-schema/
...
(examples/encryption: line   19) broken    https://dochub.mongodb.org/core/client-side-field-level-encryption - 404 Client Error: Not Found for url: https://www.mongodb.com/docs/manual/core/security-client-side-encryption/
(examples/encryption: line   54) broken    https://dochub.mongodb.org/core/client-side-field-level-encryption-read-write-support - 404 Client Error: Not Found for url: https://www.mongodb.com/docs/manual/reference/security-client-side-query-aggregation-support/
(examples/encryption: line   40) broken    https://dochub.mongodb.org/core/client-side-field-level-encryption-mongocryptd - 404 Client Error: Not Found for url: https://www.mongodb.com/docs/manual/reference/security-client-side-encryption-appendix/#mongocryptd

Opened https://jira.mongodb.org/browse/PYTHON-3361

[juliusgeo]

LGTM!

[juliusgeo]

This is such a cool name for a function compared to a2b_hex