Explicit skeletons required

This is step one (of two) to make tests freely groupable. Removes the suite as a selector for skeletons. They all move up a level. There are no default skeletons anymore.

Anyone wishing to use a skeleton must mention it explicitly in desc. Please make sure newly added tests do so.

The new names are intuitive. For example, t/skeletons/tests/default is now called t/skeletons/upload-native.

As a side note, the test suite did not run cleanly when the MR was submitted. I believe that the regressions were unrelated (and stayed the same), but I am not sure.