The test environment needs to live somewhere else

I'm writing https://github.com/ARMmbed/mbed-util and I'm trying to **not** make it depend on `mbed-core`. The problem is that I need to write tests for that library, so now I have to use this:

```
"testDependencies": {
"mbed-core": "~0.2.0"
}
```

in order to get the test environment in the library. This is just a very awkward hack, not a solution. It looks like the test environment should live in its own repository. Yup, another repository.

Start with mbed-util's testDependencies entry, which currently pulls in mbed-core, and trace how that provides the test environment. Define the separate test-environment repository boundary and verify that mbed-util can run its tests without depending on mbed-core.

Written by the indexing model from the issue text.