Document best practices

Here is a draft for what the best practices should contain. Follow-up of #18.

Topics

No console.log() on non-primitive values in tests

As @mjhea0 pointed out, the console.* methods will keep a reference to any object/array/function you pass them, thus artificially introducing a memory leak. See http://stackoverflow.com/a/28859493/1799408.

Leakage tests are quite slow

... so think twice before running it together with your other tests everytime. Point out why they are quite slow (as in the FAQ/Timeouts). Ideally present alternatives.

Make test fail first, then fix it

Similar to other kinds of test it is also best practice to first check that your leakage test actually works, by purposely introducing a leak and watching the test fail. Remove the initial leak again and make sure the test passes.

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