test: bump threshold to avoid spurious failure for possibly slow CI

[trentm]

This failed in GH Actions CI:
not ok 16 captureError is fast (<5ms): 6.238244ms

[trentm]

This had failed in https://github.com/elastic/apm-agent-nodejs/actions/runs/4552294468/jobs/8027446510

        node_tests_1     | ok 15 apm.captureError still calls back with an error id: d563b7ab8fcfa95ee523347fb13acce4
        node_tests_1     | not ok 16 captureError is fast (<5ms): 6.238244ms
        node_tests_1     |   ---
        node_tests_1     |     operator: ok
        node_tests_1     |     expected: true
        node_tests_1     |     actual:   false
        node_tests_1     |     at: <anonymous> (/app/test/context-propagation-only.test.js:195:9)
        node_tests_1     |     stack: |-
        node_tests_1     |       Error: captureError is fast (<5ms): 6.238244ms

[apmmachine]

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• Start Time: 2023-03-29T20:57:22.290+0000

• Duration: 21 min 44 sec

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