GODRIVER-1530 GODRIVER-1213 Fix the enterprise auth test runner.

[matthewdale]

GODRIVER-1530, GODRIVER-1213

Summary

• Refactor the enterprise auth test runner to use mongo.Connect instead of topology.New.
• Load the MongoDB test URI from the MONGODB_URI environment variable and ensure the URI environment variable is always passed to the test runner.
• Move the test runner to the cmd/ directory. Remove all outdated examples from the x/mongo/driver/examples directory.
• Allow the SERVICE_HOST auth mechanism property to be specified in a connection string.

Background & Motivation

#1044 broke the enterprise auth tests, which are only run on the waterfall and went unnoticed until now. That PR broke the tests because it didn't pull the database name from the connection string, instead it used "test" for all tests. In any case, we shouldn't use the unstable topology.Topology APIs for the enterprise auth tests anyway because it's not representative of how most people use enterprise auth (most use the mongo.Client type via mongo.Connect).

Refactor the enterprise auth test runner to use mongo.Connect instead of topology.New, loading the MongoDB test URI from the MONGODB_URI environment variable and using the defaultauthdb database name from the connection string, and move the test runner to the cmd/ directory. Remove all outdated examples from the x/mongo/driver/examples directory.

[benjirewis]

Evergreen changes and the new testentauth/main.go LGTM.

Those examples definitely look outdated; I think I'm fine removing, as I don't see what to replace the examples with. But, can we also close GODRIVER-2527 and remove the x/mongo/driver/examples folder entirely?

[prestonvasquez]

LGTM

[benjirewis]

LGTM assuming tests pass, thank you.