Add categories to crates.io metadata

serde_test/Cargo.toml

@@ -3,6 +3,7 @@ name = "serde_test"
 version = "1.0.140" # remember to update html_root_url
 authors = ["Erick Tryzelaar <erick.tryzelaar@gmail.com>", "David Tolnay <dtolnay@gmail.com>"]
 build = "build.rs"
+categories = ["development-tools::testing"]
 description = "Token De/Serializer for testing De/Serialize implementations"
 documentation = "https://docs.serde.rs/serde_test/"
 homepage = "https://serde.rs"