Run exercises tests in a single sbt session

[grzegorz-bielski]

Currently, the tests (bin/test) are really slow as they start a new sbt shell for each exercise. In the CI this takes around 20 minutes (!).

Ideally this should be refactored so the sbt is initialized only once and each exercise is its own submodule.

In addition current testing approach has following problems:

• It's not portable, the bin/test doesn't work on Mac.
• Uses sed for stripping out pending from all exercises. Perhaps we can somehow do it from the Scala side instead and in general rethink our approach here (see forum thread)

I believe it to be a blocker for further development of the track, since it makes the dev feedback loop very slow.