attempt to reduce flakiness of trace exporter shutdown tests

[ekump]

What does this PR do?

Attempts to reduce the likelihood of the trace exporter shutdown unit tests flaking on CI by:

• Directly checking AGENT_INFO_CACHE to be Some instead of checking the mock /info endpoint is hit. This is a potential race condition in tests as there is a non-zero amount of time between the endpoint being hit, the cache being populated, and the exporter checking the cache to know if it should enable the stats worker.
• Added a test-only function to check if stats computation is enabled and the worker is running. In tests it may be possible that we start the shutdown before stats worker is active.

Motivation

Flaky tests aren't cool

Additional Notes

Anything else we should know when reviewing?

How to test the change?

Describe here in detail how the change can be validated.