DOCS: add short writeup about testing strategies, etc.

Author: MKaczkow

testing/README.md

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+# Testing
+
+## Key Terms:
+* observability
+* controlability
+* isolateability
+* automatability
+* understandability
+![testing key terms](./testing-1.png)
+
+## Types
+* state-based tests
+* interaction tests
+
+## Strategy 
+* expression
+$GIVEN \dots WHEN \dots THEN \dots$
+* evaluation criteria
+  * `confidence` provided by tests
+  * `speed`
+  * `maintainability`
+* artifacts
+* `stub` -> allow dependency isolation, but are not themselves subjects of verification (present in GIVEN, but not in THEN)
+* `mock` -> allow dependency isolation and are themselves subjects of verification (present in GIVEN and in THEN)
+
+## Practical
+* striking balance between unit tests (domain-driven, high speed, low confidence) and integration tests (test cases and dependencies, low speed, high confidence)
+* system tests are low speed, low maintainability, high confidence
+* GUI tests are low speed, low maintainability, high confidence
+![testing summary](./testing-2.png)
+
+# References
+* [automated tests why](https://kamilgrzybek.com/blog/posts/automated-tests-the-why)
+* [automated tests testability](https://kamilgrzybek.com/blog/posts/automated-tests-testability)
+* [automated tests strategy](https://kamilgrzybek.com/blog/posts/automated-tests-strategy)