Run additional tests by using feature flag to "cargo test"

I have some tests that I would like to ignore when using cargo test (to enable offline development) and only run when explicitly passed a feature flag.

I know this can be done by using #[ignore] and cargo test -- --ignored, but I'd like to have other ignored tests for other reasons.

I have tried this:

#[test]
#[cfg_attr(not(feature = "online_tests"), ignore)]
fn get_github_sample() {
}

This is ignored when I run cargo test as desired.

But I can't get it to run, and have tried:

cargo test --features "online_tests"

cargo test --all-features

But they continue to be ignored.

I then added the feature definition into my Cargo.toml as per this page, but they continue to be ignored.

I am using workspaces in cargo. I tried adding the feature definition in both Cargo.toml files, no difference.

Any ideas?