completions command

The completions command is meant for dump terminals that cannot use
the default tab completion. It has been built for use by the emacs
sbt-mode (see https://github.com/hvesalai/sbt-mode), but is equally
useful for other code editors that can integrate with sbt.
Completion command: support quoted strings
Third draft to execute the forked tests in parallel.

This feature is not activated by default. To enable it set `testForkedParallel` to `true`.

The test-agent then executes the tests in a thread pool.
For now it has a fixed size set to the number of available processors.
The concurrent restrictions configuration should be used.
made fork-parallel scripted test a little more robust