Document the Exercism testing philosophy

contributing-to-language-tracks/exercise-test-suites.md

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+At the most basic level, Exercism is all about the tests. They drive the user's implementation forward and tell them when the exercise is complete.
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+The utmost care and attention should be used when adding or making changes to the tests for an exercise. When implementing an exercise test suite, we want to provide a good user experience for the people writing a solution to the exercise. People should not be confused or overwhelmed.
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+We simulate Test-Driven Development (TDD) by implementing the tests in order of increasing complexity. We try to ensure that each test either
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+- helps triangulate a solution to be more generic, or
+- requires new functionality incrementally.
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+The characteristics of a good individual test are:
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+- a clear name or description that reveals the intent of the test
+- a minimal amount of setup
+  - Temporary variables should be avoided unless they add to the test's clarity.
+  - Well named test helpers can be employed to encapsulate shared setup between tests.
+  - Test helpers should be extremely simple, and branching logic inside a helper is to be avoided.
+- a manageable increase in complexity from the previous passing test 
+- introduces a single new requirement
+- a minimal number of expectations required to specify the change (one expectation is preferred)
+- a readable and understandable failure message
+- adherence to the testing idioms of the language and testing framework of the track
+- does not duplicate the expectations of another test
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+The test suite of an exercise should be guided by the common specifications laid out by the `canonical-data.json` file of each exercise in [x-common](https://github.com/exercism/x-common) if it is available. The specifications there provide good defaults for:
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+- the order of the tests
+- the descriptions of the tests
+- the inputs and expected outputs of each test
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+Feel free to implement a test generator for your track if you feel it would aid in creating test suites from `canonical-data.json`.
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+The specifications in `canonical-data.json` are the strongly recommended defaults, but if you have a good reason (language idioms, etc) then don't hesitate to deviate from them.
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+If there is no `canonical-data.json` file for the exercise then we recommend using  http://x.exercism.io/problems/hello-world (replace hello-world with the exercise in question) or http://exercism.io/languages/vimscript/todo (replace vimscript with your track) to find other implementing tracks.