I noticed a relatively serious issue when testing a config with a custom gatherer. We always run the gatherer after all our default config gatherers which means that FullPageScreenshot and BFCacheFailures can mess with any instrumentation or results that the custom gatherer is using.

This PR adds an internal priority factor that we can use to sort artifacts so that custom gatherers will run always run before FullPageScreenshot and BFCacheFailures.

It might be useful to add this priority factor to the config so it isn't so subtle, but I wanted to avoid new config spec changes this late into the process.

Some other ideas:

• Tell people to use usePassiveGathering: true if they have a custom gatherer
• Instead of doing a priority factor, add an isDestructive flag