Update docs for testing

Author: wtdcode

docs/Testing.md

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+# Testing in Unicorn
+
+Unicorn focus on the testing to reduce bugs and ensure the expected behaviors. The `tests/` directory contains a few subdirectories for testing purposes.
+
+- `unit/`: C unit tests since Unicorn 2.
+- `regress/`: The regression tests written in Python and C, imported from Unicorn 1.
+- `rust-tests/`: The tests written in rust.
+- `fuzz/`: The fuzz drivers for OSS-Fuzz.
+- `benchmarks/`: The benchmark suite imported from Unicorn 1.
+
+## Contribution Guide
+
+Generally, it is ideal to add new tests whenever a PR is made. `unit/` should be the first place for the new tests to go.

tests/README.md

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+# Testing
+
+See [this](../docs/Testing.md)