test(chrome-plugin): always run "headless" when testing + increase workers

[elijah-potter]

Description

While debugging a failing snapshot test in the Chrome plugin, I realized there are some simple things that can be done to make the testing experience in the Chrome plugin a tad better:

1. Force it to always run headless on Linux. I'm sure there's a way to do this on other platforms, but this one is easy.
2. Increase the number of workers are spawned per run. This change does not affect CI, only local platforms.

How Has This Been Tested?

Manually.

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.