Add some TestGame files to .gitignore (#2465)

Author: ike709

.gitignore

@@ -1,4 +1,4 @@
-## Ignore Visual Studio temporary files, build results, and
+## Ignore Visual Studio temporary files, build results, and
 ## files generated by popular Visual Studio add-ons.
 
 # User-specific files
@@ -322,3 +322,7 @@ ehthumbs.db
 # JetBrains Rider
 .idea/
 *.sln.iml
+
+# OpenDream-generated files for TestGame
+TestGame/preprocessor_dump.dm
+TestGame/environment.dmd