fix: Make TestChunksIntervalSeconds less flaky

[DrJosh9000]

Description

Fix a flaky test.

Context

https://buildkite.com/organizations/buildkite/analytics/suites/buildkite-agent/tests/8503693e-4ff1-8a30-9887-f177a8175585?query=TestChunksIntervalSeconds

Changes

Check for a range of possible values, and update the comments.

A few other flourishes while I'm here:

• Split the one subtest into two. Instead of a wait group, use t.Run twice with t.Parallel. Because the two values are not actually compared against each other, even though that's what the old subtest name implies.
• Use an atomic - this isn't addressing any problem, a mutex is fine. It just feels more right.

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

Spite and keyboards.