Add test for signals

[jprendes]

This PR adds test for sending signals to containers.
This is the first step towards fixing #755.

Running:

USE_LIBC=0 cargo test -p containerd-shim-wasm -- test::signals::test_handling_signals --exact --show-output --nocapture --ignored

is very likely to stall

While running:

USE_LIBC=1 cargo test -p containerd-shim-wasm -- test::signals::test_handling_signals --exact --show-output --nocapture --ignored

does succeed without problems.

The main difference is that the first one uses tokio::signal, while the second one uses libc::signal.

Commenting out the containerd::Client code in crates/containerd-shim-wasm/src/sys/unix/container/instance.rs prevents tokio from being used in the shim, and both versions of the test pass reliably.

[Mossaka]

Great to see more tests being added!

I do believe this change should be appended as a "Added" section to the CHANGELOG since it's adding a new test feature.

[Mossaka]

LGTM!