LigthouseRunWarnings are not completely deduplicated

[brendankenny]

Noticed by @connorjclark over in googleads/publisher-ads-lighthouse-plugin#232

There are two problems at work here:

1. LighthouseRunWarnings are deduplicated as an artifact in gatherRunner (and there's a test for this), but when they were extended to let audits add warnings, the audit warnings were never deduped. I believe this is what's going on with lighthouse-plugin-publisher-ads.
2. I don't believe this second problem has been possible yet, but it soon will be. As mentioned, LighthouseRunWarnings are deduped in gatherRunner, but they're deduped as primitive values. With the introduction of LH.IcuMessage in i18n: use IcuMessage objects instead of string IDs #10630, Array.from(new Set(baseArtifacts.LighthouseRunWarnings)) will not effectively dedupe them, since two identical LH.IcuMessages will usually not be the same instance, even if they result in the same warning string. Wouldn't be difficult to use the IcuMessage shape to dedupe directly, but it might be easiest to do something with lodash.isequal since it's already in our bundle.