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

default.nix

@@ -1,10 +1,11 @@
-{ 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
     ];

platformio.ini

@@ -157,6 +157,7 @@ lib_deps =
 
 [env:native]
 platform = native
+test_framework = googletest
 build_flags = -std=c++17
   -I src
   -I test/mocks