chore: import upstream snapshot with attribution

python/ray/data/tests/unit/README.md

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+# Unit Tests
+
+This directory contains unit tests that do not depend on distributed infrastructure or external dependencies.
+
+## Requirements
+
+Unit tests in this directory must be:
+- **Fast**: Execute in milliseconds, not seconds
+- **Isolated**: No dependencies on Ray runtime, external services, or file I/O
+- **Deterministic**: No randomness or time-based behavior
+
+## Restrictions
+
+Tests should NOT:
+- Initialize or use the Ray distributed runtime
+- Use `time.sleep()` or other time-based delays
+- Depend on external services, databases, or file systems
+- Make network calls
+
+## Enforcement
+
+The `conftest.py` in this directory enforces these restrictions by preventing:
+- `ray.init()` from being called
+- `time.sleep()` from being used
+
+If a test requires any of these, it should be moved to the main test directory instead.