ts: new standalone pipeline test and slight improvements
Implement a test that initializes pipelines with minimalistic theadshare src and sink. This can help with the evaluation of changes to the threadshare runtime or with element implementation details. It makes it easy to run flamegraph or callgrind and to focus on the threadshare runtime overhead.
Using callgrind with this test showed opportunities for improvements for sub tasks addition and drain. See second commit for details.