Clear CARGO_TARGET_DIR for tests.

tests/support/client/mod.rs

@@ -59,6 +59,10 @@ impl Project {
         let mut cmd = Command::new(rls_exe());
         cmd.current_dir(self.root());
         cmd.stderr(Stdio::inherit());
+        // If `CARGO_TARGET_DIR` is set (such as in rust-lang/rust), don't
+        // reuse it. Each test needs its own target directory so they don't
+        // stomp on each other's files.
+        cmd.env_remove("CARGO_TARGET_DIR");
 
         cmd
     }