Remove module-level tests directories, keep only main tests/ for exported package validation

- Remove all tests/ directories under modules/source/
- Keep main tests/ directory for testing exported functionality
- Update status command to check tests in main tests/ directory
- Update documentation to reflect new test structure
- Reduce maintenance burden by eliminating duplicate test systems
- Focus on inline NBGrader tests for development, main tests for package validation

pyproject.toml

@@ -84,7 +84,6 @@ addopts = [
 ]
 testpaths = [
     "tests",
-    "modules/*/tests",
 ]
 python_files = ["test_*.py"]
 python_classes = ["Test*"]