Integrate new unified spec tests for load balancers

New integration tests written in the unified test format to validate LB support behavior. The "source/load-balancers/tests/README.rst" file contains information for setting up load-balanced sharded clusters in Evergreen and lists which sets of tests must be run. See https://github.com/mongodb/specifications/commit/da077ac0b0df421c00fa7f8fc63b1d291bb5b9d5 for relevant changes.