Fix function $0 param, scope positional args locally, follow symlinks in completion, expose SHED_VI_MODE variable, and clean up test helpers

flake.nix

@@ -22,9 +22,12 @@
         lockFile = ./Cargo.lock;
       };
 
-      doCheck = false;
       passthru.shellPath = "/bin/shed";
 
+      checkPhase = ''
+        cargo test -- --test-threads=1
+      '';
+
       meta = with pkgs.lib; {
         description = "A Linux shell written in Rust";
         homepage = "https://github.com/km-clay/shed";