Move testing strategy to a dedicated doc section (fix #114)

doc/book/working-documents/design-draft.md

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 title = "Design draft (obsolete)"
-weight = 50
+weight = 900
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 **WARNING: this documentation is a design draft which was written before Garage's actual implementation.

doc/book/working-documents/load-balancing.md

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 title = "Load balancing data (obsolete)"
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+weight = 910
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 **This is being yet improved in release 0.5. The working document has not been updated yet, it still only applies to Garage 0.2 through 0.4.**

doc/book/working-documents/testing-strategy.md

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+title = "Testing strategy"
+weight = 30
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+
+
+## Testing Garage
+
+Currently, we have the following tests:
+
+- some unit tests spread around the codebase
+- integration tests written in Rust (`src/garage/test`) to check that Garage operations perform correctly
+- integration test for compatibility with external tools (`script/test-smoke.sh`)
+
+We have also tried `minio/mint` but it fails a lot and for now we haven't gotten a lot from it.
+
+In the future:
+
+1. We'd like to have a systematic way of testing with `minio/mint`,
+   it would add value to Garage by providing a compatibility score and reference that can be trusted.
+2. We'd also like to do testing with Jepsen in some way.
+
+## How to instrument Garagae
+
+We should try to test in least invasive ways, i.e. minimize the impact of the testing framework on Garage's source code. This means for example:
+
+- Not abstracting IO/nondeterminism in the source code
+- Not making `garage` a shared library (launch using `execve`, it's perfectly fine)
+
+Instead, we should focus on building a clean outer interface for the `garage` binary,
+for example loading configuration using environnement variables instead of the configuration file if that's helpfull for writing the tests.
+
+There are two reasons for this:
+
+- Keep the soure code clean and focused
+- Test something that is as close as possible as the true garage that will actually be running
+
+Reminder: rules of simplicity, concerning changes to Garage's source code.
+Always question what we are doing.
+Never do anything just because it looks nice or because we "think" it might be usefull at some later point but without knowing precisely why/when.
+Only do things that make perfect sense in the context of what we currently know.
+
+## References
+
+Testing is a research field on its own.
+About testing distributed systems:
+
+ - [Jepsen](https://jepsen.io/) is a testing framework designed to test distributed systems. It can mock some part of the system like the time and the network.
+ - [FoundationDB Testing Approach](https://www.micahlerner.com/2021/06/12/foundationdb-a-distributed-unbundled-transactional-key-value-store.html#what-is-unique-about-foundationdbs-testing-framework). They chose to abstract "all sources of nondeterminism and communication are abstracted, including network, disk, time, and pseudo random number generator" to be able to run tests by simulating faults.
+ - [Testing Distributed Systems](https://asatarin.github.io/testing-distributed-systems/) - Curated list of resources on testing distributed systems
+ 
+About S3 compatibility:
+  - [ceph/s3-tests](https://github.com/ceph/s3-tests)
+  - (deprecated) [minio/s3verify](https://blog.min.io/s3verify-a-simple-tool-to-verify-aws-s3-api-compatibility/)
+  - [minio/mint](https://github.com/minio/mint)
+
+About benchmarking S3 (I think it is not necessarily very relevant for this iteration):
+  - [minio/warp](https://github.com/minio/warp)
+  - [wasabi-tech/s3-benchmark](https://github.com/wasabi-tech/s3-benchmark)
+  - [dvassallo/s3-benchmark](https://github.com/dvassallo/s3-benchmark)
+  - [intel-cloud/cosbench](https://github.com/intel-cloud/cosbench) - used by Ceph
+  
+Engineering blog posts:
+ - [Quincy @ Scale: A Tale of Three Large-Scale Clusters](https://ceph.io/en/news/blog/2022/three-large-scale-clusters/)
+
+Interesting blog posts on the blog of the Sled database: 
+
+- <https://sled.rs/simulation.html>
+- <https://sled.rs/perf.html>
+  
+Misc:
+  - [mutagen](https://github.com/llogiq/mutagen) - mutation testing is a way to assert our test quality by mutating the code and see if the mutation makes the tests fail
+  - [fuzzing](https://rust-fuzz.github.io/book/) - cargo supports fuzzing, it could be a way to test our software reliability in presence of garbage data.
+  
+