`analyzer_testing` testing for built-in rules

[FMorschel]

While doing some testing on the new plugin system, I discovered that we could not test whether Dart's built-in rules are active or not. Since the built-in rules are not registered for the default setup. So we either need to create a dummy rule with the same name, or we can't test the default values.

But I believe the analyzer_testing package should already include the built-in dummy rules for testing. So we don't have to do more work than necessary.

Also, we could possibly add a tutorial on how to test the built-in rules and consider them for our plugin rules.

CC @srawlins @bwilkerson