Support CI coverage of non-default features (i.e., `bevy_asset/http`).

Some features we intentionally don't enable by default. This allows users to opt in to these more niche features, and only when they need them.

Unfortunately, this comes with a big problem: you can write all the tests you want for your new feature, and CI won't run those tests at all. This can result in functionality or tests being broken.

For a concrete example, on current main `bevy_asset/http` fails to compile, and yet CI happily says everything is fine, despite the fact we have tests for these. In a [recent PR](https://github.com/bevyengine/bevy/pull/22520#discussion_r3025672792), this same issue came up where I was worried that changes to `bevy_ecs` could result in this change failing to compile, and silently rotting.

We need a better solution here! Perhaps something like providing some specific configurations that we know are relevant to test for optional features.