feat: add internal/testing-only opts._elasticApm to facilitate APM agent tests

[trentm]

Expected usage in APM agent tests is something like:

const apm = require('../..')  // note the import path is not "elastic-apm-node"
const log = pino(ecsFormat({ _elasticApm: apm }))
// ...

This allows the APM agent tests to use the local APM agent working copy to test APM tracing integration in the logger.
This is going to be used to test elastic/apm-agent-nodejs#2127

[apmmachine]

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• Build Cause: Pull request #84 updated

• Start Time: 2021-06-28T20:12:51.573+0000

• Duration: 8 min 13 sec

• Commit: c5a9c81

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[trentm]

@elastic/[email protected] published