Fix flaky vega screenshot test

[lukeelmers]

#32749 introduced a screenshot-based functional test for validating the output of vega visualizations. this test was subsequently skipped in #33074 due to flakyness.

most other functional tests for visualizations rely on tabular data from the inspector to assert that the right data are being displayed, but since vega doesn't have inspector, this isn't an option.

the other place we are are using screenshot tests is in the dashboard, where we have the option of using full screen mode to maintain focus on the visualization without worrying about things like the nav evolving over time. unfortunately, full screen isn't an option within the visualize editor.

what we've tried:

1. (current approach) inject some CSS to the DOM, hiding unnecessary elements, and then take the screenshot. this passes consistently in Chrome on OSX, but when run in CI will usually fail.
2. inspect the DOM, find the vega canvas element, call canvas.toDataUrl() and compare the base64-encoded output. in initial testing, this proved problematic as it seemed rendering the vis with the same filters didn't always reproduce the exact same encoded output... though might be worth revisiting to test further.
3. messing with the variance threshold for the screenshot comparison. this is a challenge because it needs to be sensitive enough to detect small differences in line charts, and raising the threshold too high will result in false positives.

[elasticmachine]

Pinging @elastic/kibana-test-triage

[elasticmachine]

Pinging @elastic/kibana-app

[flash1293]

@lukeelmers This is probably not super present (given it's more than a year old), but I still want to ask whether you tried using the svg renderer for vega ("config": { "kibana": {"renderer": "svg"} } as part of the spec). Then we can validate whether the svg elements are set up correctly (with some allowance for sub pixel differences as well).

If you didn't try that, I'm happy to pick up the task.

[lukeelmers]

I still want to ask whether you tried using the svg renderer for vega ("config": { "kibana": {"renderer": "svg"} } as part of the spec

Nope, definitely did not consider this approach, so feel free to pick it up -- Thanks @flash1293!