[Gaxios] browser-test are not running/working.

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### Link to the code that reproduces this issue. A link to a **public** Github Repository or gist with a minimal reproduction.

https://github.com/googleapis/google-cloud-node-core

### A step-by-step description of how to reproduce the issue, based on the linked reproduction.

npm run browser-test

### A clear and concise description of what the bug is, and what you expected to happen.

Migrating gaxios to this repo, notice browser-test are not running and working locally.

### A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

browser test should run and pass.