test: add some context to test failure to help with debugging it

[trentm]

A tweak to get some details if this test (sensitive to timing) fails again in CI.

There was a recent failure of this test in GH Actions CI: https://github.com/elastic/apm-agent-nodejs/runs/6081847063?check_suite_focus=true#step:8:2934
It might be helpful to know if span.composite.sum was exactly 30 for that failure, or less, to know how to best make this test less flaky.

[apmmachine]

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• Start Time: 2022-04-19T18:03:28.575+0000

• Duration: 27 min 56 sec

Test stats 🧪

Test	Results
Failed	0
Passed	244817
Skipped	0
Total	244817

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