Add consensus-spec-tests to Testing

docs/wiki/testing/consensus-spec-tests.md

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+# Consensus-spec tests
+
+> :warning: This article is a [stub](https://en.wikipedia.org/wiki/Wikipedia:Stub), help the wiki by [contributing](/contributing.md) and expanding it.
+
+The [consensus-spec-tests](https://github.com/ethereum/consensus-spec-tests) repository contains a collection of test vectors generated from the Ethereum consensus specifications.  
+These tests are used to verify the correctness of consensus client implementations against the [Ethereum consensus-specs](https://github.com/ethereum/consensus-specs).
+
+Client developers use these tests to confirm that changes to the protocol are implemented consistently across all consensus clients.  
+
+---
+
+> ### Practical notes
+>
+>The `consensus-spec-tests` repository is quite large and can take several gigabytes of disk space.  
+When running multiple consensus clients that rely on the same test vectors, it is technically possible to save space by creating **symlinks** from each client’s expected test directory to a single shared copy of `consensus-spec-tests`.
+>
+>However, this approach comes with risks:
+>- Different clients may depend on **different versions** of the test suite.  
+  Using the wrong version can result in **false negatives** (tests failing even though the client implementation is correct).
+    ![consensus-spec-tests-different-versions](./img/consensus-spec-tests-different-versions.png)
+>    
+>- Manually switching (`git checkout`) to the required test version for a client and rerunning the tests can cause **issues with unpacking SSZ Snappy files** and lead to failing tests.
+    ![consensus-spec-tests-incorect-unpacking-SSZ](./img/consensus-spec-tests-incorect-unpacking-SSZ.png)
+> See [issue](https://github.com/sntntn/grandine/issues/15) for a concrete example.
+>
+> Because of this, the **recommended practice** is to **keep separate copies** of `consensus-spec-tests` for each consensus client, even if it requires more disk space.
+
+
+
+
+Resources:  
+- [consensus-spec-tests repo](https://github.com/ethereum/consensus-spec-tests)  
+- [Ethereum consensus-specs](https://github.com/ethereum/consensus-specs) (source of truth for test generation)  
+- [SimpleSerialize (SSZ) specification](https://ethereum.github.io/consensus-specs/ssz/simple-serialize/)  
+