Tweak Karma CI options
Tweak Karma CI options

* Limit SauceLabs concurrency on CI.
* Increase RetryLimit on CI.

I'm seeing lots of intermittent failures on CI. I ssh'd into a running build and tweaking the concurrency and retryLimit  options seemed the most reliable way to ensure builds don't fail due to sporadic failures.

I picked 3 as the concurrency as any higher made the tests fail more often. I increased the retryLimit to 3 as I do see browsers failing to start quite often (at least once every build), and retrying more than twice for such a common failure seemed like a good idea. We could go higher, but I didn't have tests fail even once with retrying thrice.

Limiting concurrency will make CI builds slower. I think this is ok because this repo isn't that frequently updated, and I think this'll save us in the long run by not having engineers waste time on fixing CI.

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