Fix wheel testing

[kbonney]

Summary

Wheel testing was failing because of issues with the filepaths declared in conftest.py.
This PR addresses this issue by making some minor tweaks to the test suite configuration:

• Removed wntr/tests/init.py — tests are no longer a subpackage, so they don't get bundled into the wheel
• Moved shared path constants out of conftest.py into a new _test_paths.py utility module. conftest is now purely pytest infrastructure (backend, CWD isolation), while path constants live somewhere that makes their purpose obvious
• Updated all 40 test files to import from _test_paths instead of wntr.tests.conftest. This also fixes the CI wheel-test failure, since bare import _test_paths resolves from the local checkout rather than the installed package

Tests and Documentation

AI Disclosure

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