test(NODE-5855): unskip transaction tests

[nbbeeken]

Description

What is changing?

• Unskip TXN tests now that we've updated the unified spec tests
• Unskip TXN tests on LB
• Move session clearing of connection after checkIn

Also resolves: NODE-5931

Is there new documentation needed for these changes?

What is the motivation for this change?

• Testing TXN

Release Highlight

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[baileympearson]

The serverless test failures in this PR are unrelated.