ci: Attempt at making puppeteer tests more stable

Created by: jhchabran

After inspecting the failures, https://buildkite.com/sourcegraph/sourcegraph/builds/125306 (see also this graph ), display failures are ruled out. Instead, the browser appears to die or simply close the tab without any obvious reason. Correlating the memory usage which often reaches 100% on these steps, it's possible that's what is causing those issues.

Before considering increasing the allocated memory, we can split the tests to lessen the memory consumption of a given run. So rather than ending up with 6 puppeteer chunks, we now have 9. I want to observe the failure rate on the chunks in the upcoming days to see if that improves the stability.