- remove the output that is always present
- add a test that has no outputs

from planemo the verify_tool function is called from
[within an try-except block](https://github.com/galaxyproject/planemo/blob/1aa3eb05a97ad20c0be6f6560ab5cec090e76612/planemo/engine/galaxy.py#L109)
which silently catches any exception.

Thus any exception raised from within verify_tool will not be
detected, i.e. the assertion needs to be moved into
`_verify_outputs` (which also seems to make sense by name)
in order to make verify_tool record the problem properly.

thereby we implicitly make the assumption that a test needs to produce
an output

also make message more specific

otherwise the tool test will just return a list out of bounds exception
which is unclear to the user

Co-authored-by: Marius van den Beek <m.vandenbeek@gmail.com>

then it might be clearer what's going on here