Avoid quitting and relaunching the browser on each file.

[eyebrowsoffire]

Currently, the framework unit tests quit and relaunch Chrome on each file that is tested. This makes things slow, and I suspect it is a source of flakiness. Instead, we should just launch the browser as needed, and reuse the browser instance across all tests in the suite (which is what we do in the engine unit tests).

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[eyebrowsoffire]

This doesn't seem to work well, and we've found a separate fix that we think should resolve the flakiness: #135521