build: add gcc, gtest to nix shell, fix native test framework

default.nix

@@ -1,11 +1,12 @@
-{ pkgs ? import <nixpkgs> {} }:
-let
+{pkgs ? import <nixpkgs> {}}: let
 in
   pkgs.mkShell {
     buildInputs = [
       pkgs.platformio
       pkgs.python3
+      pkgs.gcc
+      pkgs.gtest
       # optional: needed as a programmer i.e. for esp32
       pkgs.avrdude
     ];
-}
+  }