fix(core): add missing tokio test features so crates build on their own

[Lstarsky0]

Which issue does this PR close?

None. The opendal-core line also exists on #8042, a much larger transport change that is currently conflicting; this splits the build fix out so it can land on its own. The second crate is not covered there.

Rationale for this change

Two crates cannot have their tests built on their own.

cargo test -p opendal-core --no-run, on any non-wasm target:

error[E0432]: unresolved import `tokio::time`
   --> core/src/types/execute/executors/tokio_executor.rs:38:16
    |
 38 |     use tokio::time::sleep;
    |                ^^^^ could not find `time` in `tokio`

Ten sites across seven files use tokio::time, all of them inside mod tests, but time is only enabled under [target.'cfg(target_arch = "wasm32")'.dependencies].

cargo check -p opendal-layer-observe-metrics-common --tests:

error: The #[tokio::test] macro requires rt or rt-multi-thread.
    --> layers/observe-metrics-common/src/lib.rs:1538:5
     |
1538 |     #[tokio::test]
     |     ^^^^^^^^^^^^^^

Its dev-dependency asks for ["macros"] alone. The other fifteen layer crates with a tokio dev-dependency all ask for a runtime as well.

Core CI runs --workspace --all-features, so feature unification supplies both from sibling crates and neither gap is visible there. They only show up when a crate is built by itself.

What changes are included in this PR?

"time" for opendal-core, "rt-multi-thread" for opendal-layer-observe-metrics-common. taplo format expanded the first array, which crossed 80 columns.

I ran cargo check -p <crate> --tests over all 105 workspace members. Nothing else fails, apart from opendal-service-hdfs and opendal-service-rocksdb, which need system libraries I do not have locally.

Are there any user-facing changes?

No. Dev-dependencies are not part of the published build, and Cargo.lock is unchanged. cargo test -p opendal-core now runs 183 unit tests and 130 doc tests, and opendal-layer-observe-metrics-common runs 18.

AI Usage Statement

AI assisted with investigating the build failures and drafting this description. I reviewed and take responsibility for the changes. There are no known assumptions or unknowns that affect review.