CI doesn't run all tests

[pcpthm]

When a Cargo workspace contains both the root package and other workspace members, cargo test by default runs tests only of the root package. In fact, i3ipc-types test fails since #7.

cargo test --workspace can be used to run tests on all workspace members as well as the root package but there is an issue. Trying to build the whole workspace at once fails due to a compilation error. That is because Cargo tries to build the whole workspace with both async-traits and async-std-traits features enabled, and because those features are not additive (see rust-lang/cargo#4328) it breaks. It should probably be a separate issue, though.

As a workaround, running cargo test in each directory will build each package separately, and doesn't merge features.