chore(ci): add goleak to TestMain

[dnwe]

Whilst this isn't as helpful as a per test defer goleak.VerifyNone in terms of showing which test was responsible for the leak, it's still worth having as a baseline check in PRs and CI to prevent new leaks being introduced now that we're all cleaned up. If a PR fails we can subsequently run tests individually in order to determine which one is the leaker.

See https://github.com/uber-go/goleak#quick-start and https://github.com/uber-go/goleak#determine-source-of-package-leaks

[github-actions[bot]]

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