[Test]: Add is_runtime_youki() helper to contest test framework for consistent skip logic

[nayuta723]

Background

When working on a FEATURE issue, it is often useful to split the work into separate PRs — one for adding the tests and one for the feature implementation itself. This allows tests to be merged early (guarded with TestResult::Skipped) and then re-enabled when the feature PR lands.

Currently, is_runtime_runc() exists in tests/contest/contest/src/utils/support.rs to skip tests when running against runc. There is no equivalent for youki, making it inconsistent and forcing ad-hoc checks like get_runtime_path().ends_with("youki") inline in test functions.

Feature Request

Add is_runtime_youki() to support.rs, mirroring is_runtime_runc(), and establish a convention of using ConditionalTest with this helper for all youki-specific skip logic.

Is the request related to some problem running youki?

Proposed Solution

Add the following to tests/contest/contest/src/utils/support.rs:

pub fn is_runtime_youki() -> bool {
    match std::env::var("RUNTIME_KIND") {
        Err(_) => false,
        Ok(s) => s == "youki",
    }
}

Usage with ConditionalTest:

ConditionalTest::new(
    "my_youki_specific_test",
    Box::new(|| !is_runtime_youki()), // skip when youki
    Box::new(|| { /* test body */ }),
)

Considerations

Additional Context