gateware: create vcd_out directory by default

gateware/.gitignore

@@ -4,4 +4,5 @@ __pycache__
 
 # Sim artifacts
 *.vcd
-*.gtkw
+*.gtkw
+vcd_out/

gateware/main.py

@@ -9,6 +9,7 @@ from minerva.core import Minerva
 
 from typing import List
 from argparse import ArgumentParser
+from pathlib import Path
 import unittest
 import sys
 import os
@@ -151,7 +152,10 @@ if __name__ == "__main__":
 
     if args.test:
         if args.save_vcd:
-            os.environ.set("TEST_SAVE_VCD")
+            if not Path("vcd_out").exists():
+                os.mkdir("vcd_out")
+
+            os.environ["TEST_SAVE_VCD"] = "YES"
 
         # Super hacky... why am I doing this
         test_modules = [mod for mod in sys.modules if mod.startswith("test_")]