Warn if `.shed.yml` is not available for the tool

Hi,
It might worth to have an automatic sanity check for the existence of `.shed.yml` file for `planemo test/lint tool.xml`
This could help for not missing the tool during CI test of the PRs.

Start with the planemo test and planemo lint command entry points and reproduce the behavior using a tool.xml without an accompanying .shed.yml. Define the warning or sanity-check behavior for that case, then verify both commands handle the missing file as intended.

Written by the indexing model from the issue text.