Remove unnecessary tests/conftest.py

After further investigation, conftest.py is not needed because:

1. When tox installs the package (even with usedevelop=false), it runs
   'pip install .' which installs ALL packages found by find_packages('src')
   including both 'datapilot' and 'vendor' packages.

2. The existing tests (tests/core/**) import from 'datapilot' and work
   fine without any conftest.py manipulating sys.path.

3. The actual fix for our import issues was renaming tests/vendor/ to
   tests/test_vendor/ to avoid the directory naming conflict with
   src/vendor/, not adding conftest.py.

When the package is properly installed, both 'datapilot' and 'vendor'
are importable without needing to manually add src/ to sys.path.

Author: jontsai