Add tempfile and tmp_env as dev dependencies

The `tempfile` crate can help us set up temporary directories used for
testing, and the `tmp_env` crate can help us set temporary environment
variables when testing.

Author: ytmimi

Cargo.lock

@@ -66,6 +66,10 @@ rustc-workspace-hack = "1.0.0"
 
 # Rustc dependencies are loaded from the sysroot, Cargo doesn't know about them.
 
+[dev-dependencies]
+tempfile = "3.3.0"
+tmp_env = "0.1.1"
+
 [package.metadata.rust-analyzer]
 # This package uses #[feature(rustc_private)]
 rustc_private = true