tests: Let .assets/ pass through Polly

Created by: chrismwendt

Prior to this change, you had to run yarn build before running E2E tests so that the ui/assets directory was populated.

After this change, .assets/ will be served from the instance.

@felixfbecker Do you happen to remember why you decided to serve .assets/ from disk in https://github.com/sourcegraph/sourcegraph/pull/12130 ?

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