use newly `await`able `agent.flush()` in tests

[trentm]

Now that awaitable flush() is in there are a few usages of promisify(agent.flush.bind(agent))() that could be converted over to use it for a slight cleanup.

[Siddhu545]

Hi @trentm I'd like to take this one.

Now that agent.flush() is natively await, it makes sense to clean up the existing promisify(agent.flush.bind(agent))() usages. I’ll update those calls to use the new awai method, remove unnecessary promisify imports where applicable, and make sure everything stays functionally equivalent. Should be a small but nice readability win.