Re-add tests to the crates.io tarball

Without them, `cargo publish` failed when verifying `Cargo.toml`.

Author: jplatte

Cargo.toml

@@ -13,7 +13,7 @@ variable.
 """
 categories = ["development-tools::debugging"]
 keywords = ["logging", "log", "logger"]
-include = ["src/**/*", "LICENSE-*", "README.md", "CHANGELOG.md"]
+include = ["src/**/*", "tests", "LICENSE-*", "README.md", "CHANGELOG.md"]
 
 [workspace]
 members = [