client/testing: relax puppeteer timeout
Created by: bobheadxi
relax this timeout from one magic number (30000ms) to another (300000ms aka 5m) which is also used elsewhere. This timeout seems to be getting hit in various flakes, e.g. https://buildkite.com/sourcegraph/qa/builds/5781#64f82fac-0650-47d9-9a67-47c4f91afcb1/874-971:
sourcegraph-e2e: 1 failing
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| sourcegraph-e2e: 1) e2e test suite
| sourcegraph-e2e: "before all" hook in "e2e test suite":
| sourcegraph-e2e: TimeoutError: Navigation timeout of 30000 ms exceeded
This is a pretty frequent flake and nothing seems to be wrong when deployed, so this should be a safe change.