tests: finalize the test for dynamic threading

Looks like this was never finished. Because it was also using the old testing
API, the final untagged THREAD was not even handled...
Threading: refactoring
obs: keep the trojita-nightly versioning in sync with usptream
Let's call this the 0.3.93 -- too many changes are in already
Marking the current version as -git again
build: add .dsc for the new Ubuntu
trojita-nightly: upgrade the version number