Document test organization structure

Create root TESTING.md documenting:
- Directory structure and organization
- Separation principle (core VCS vs setuptools integration)
- How to run tests (all, core only, integration only)
- Test fixtures and shared infrastructure
- Migration notes

This completes the test reorganization plan.

Author: RonnyPfannschmidt

TESTING.md

@@ -0,0 +1,78 @@
+# Testing Organization
+
+This document describes the test organization in the setuptools-scm monorepo.
+
+## Directory Structure
+
+The repository contains two test suites:
+
+- **`vcs-versioning/testing_vcs/`** - Core VCS versioning functionality tests
+- **`setuptools-scm/testing_scm/`** - Setuptools integration and wrapper tests
+
+## Separation Principle
+
+Tests are organized by architectural layer:
+
+### Core VCS Layer (`vcs-versioning/testing_vcs/`)
+
+Tests for core version control system functionality:
+- VCS backend operations (Git, Mercurial parsing)
+- Version scheme and formatting logic
+- Configuration validation
+- Version inference
+- Error handling
+- Core utility functions
+
+**When to add tests here:** If the functionality is in `vcs_versioning` package and doesn't depend on setuptools.
+
+### Setuptools Integration Layer (`setuptools-scm/testing_scm/`)
+
+Tests for setuptools-specific functionality:
+- Setuptools hooks and entry points
+- `setup.py` integration (` use_scm_version`)
+- `pyproject.toml` reading and Configuration.from_file()
+- File finding for setuptools (sdist integration)
+- Distribution metadata
+- setuptools-scm CLI wrapper
+
+**When to add tests here:** If the functionality is in `setuptools_scm` package or requires setuptools machinery.
+
+## Running Tests
+
+### Run all tests
+```bash
+uv run pytest -n12
+```
+
+### Run core VCS tests only
+```bash
+uv run pytest vcs-versioning/testing_vcs -n12
+```
+
+### Run setuptools integration tests only
+```bash
+uv run pytest setuptools-scm/testing_scm -n12
+```
+
+### Run specific test file
+```bash
+uv run pytest vcs-versioning/testing_vcs/test_version_schemes.py -v
+```
+
+## Test Fixtures
+
+Both test suites use `vcs_versioning.test_api` as a pytest plugin, providing common test infrastructure:
+
+- `WorkDir`: Helper for creating temporary test repositories
+- `TEST_SOURCE_DATE`: Consistent test time for reproducibility
+- `DebugMode`: Context manager for debug logging
+- Repository fixtures: `wd`, `repositories_hg_git`, etc.
+
+See `vcs-versioning/src/vcs_versioning/test_api.py` and `vcs-versioning/src/vcs_versioning/_test_utils.py` for details.
+
+## Migration Notes
+
+File finders remain in setuptools-scm because they're setuptools integration (registered as `setuptools.file_finders` entry points), not core VCS functionality.
+
+For deeper unit test conversions beyond basic reorganization, see `setuptools-scm/testing_scm/INTEGRATION_MIGRATION_PLAN.md`.
+