Added code coverage configuration file

#396 in iced-rs/iced_aw — merged 2026-01-01

Summary

This is a small PR dealing with code coverage.

• I've added tarpaulin.toml to configure code coverage runs, and updated the testing guidelines with a code coverage section.
• As of my most recent analysis, code coverage is sitting at about 44% lines covered. As I state in the testing guide (tests/README.md), it is suggested that integration testing prioritize critical paths and common failure points rather than exhaustive coverage. But metrics help us to know where we stand, so at any time detailed coverage reports can be seen in the target/tarpaulin folder after running:

cargo tarpaulin