Add tests for Transform on_begin()/on_end() propagation.

Transform instances should call on_begin() and on_end() on the downstream
sink but casual testing while developing has shown they do not. Those
tests show the code is broken.


Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
Fix Transform class to call on_begin()/on_end()

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
Merge branch 'feature/fix-misc-propagation' into develop