Use new Cargo feature flag to skip "Bitcoin" tests

[obycode]

I don't like that this prevents us from using ignored to actually ignore a test. What if we take this opportunity to use a different technique for these tests, and instead of marking them all as ignored, we only mark the ones that we want to ignore as ignored, and for the others we could use a new Cargo feature flag to ensure the simple CI job ignores them?

The added benefit is that we would see directly at the source of the test that it really is being ignored.

Originally posted by @obycode in #5763 (comment)