Move shared DGP to tests/helpers/, remove global pythonpath

P3: Move edid_dgp.py into tests/helpers/ and add the helpers
directory to sys.path via conftest.py instead of the global
pythonpath = ["tests"] setting. This avoids making every module
under tests/ importable as a top-level module.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber

pyproject.toml

@@ -89,7 +89,6 @@ python-packages = ["diff_diff"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
-pythonpath = ["tests"]
 python_files = "test_*.py"
 # Exclude slow tests by default; use `pytest -m ''` to run all tests
 addopts = "-v --tb=short -m 'not slow'"

tests/conftest.py

@@ -8,6 +8,10 @@
 import math
 import os
 import subprocess
+import sys
+
+# Make tests/helpers/ importable without adding all of tests/ to sys.path
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "helpers"))
 
 # Force non-interactive matplotlib backend before any test imports it.
 # Prevents plt.show() from blocking the test suite on a GUI window.