Include tests in the crate again

…so it is possible to run checks on the crate again.

Fixup for 7a2fe90.

Author: keszybz

Cargo.toml

@@ -7,7 +7,7 @@ authors     = ["The nix-rust Project Developers"]
 repository  = "https://github.com/nix-rust/nix"
 license     = "MIT"
 categories  = ["os::unix-apis"]
-include = ["src/**/*", "LICENSE", "README.md", "CHANGELOG.md"]
+include = ["src/**/*", "test/**/*", "LICENSE", "README.md", "CHANGELOG.md"]
 
 [package.metadata.docs.rs]
 targets = [