This is the second in a series of PR's for implementing the retryable writes spec tests and updating our test runner framework.

This PR updates the test utils to be more usable for the new runner.
In total:

• moves ServerVersion out of a client extension such that it is its own type
• adds Comparable conformance to ServerVersion so we don't need to implement/use the various isGreaterThan, isLessThan, etc. methods
• adds an initializer to TopologyType for use in testing
• adds an internal sortedEquals method to a Document extension for testing purposes (so we don't need to use an expect to get sorted equality)
• moves the WriteModel typealiases into TestUtils.swift