Add targets for tests

[AndrewGaspar]

[baszalmstra]

Would this be as simple as adding:

add_test(
        NAME cargo-test_${target_name}
        COMMAND
        $<TARGET_FILE:Rust::Cargo> test --target ${_CORROSION_RUST_CARGO_TARGET}
        -p ${package_name} --manifest-path ${path_to_toml}
        WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/${build_dir}
    )

to the _add_cargo_build function?