Add known test directories as `TestDefinition` instances

[simleo]

E.g.:

{
    "@id": ".tests",
    "@type": ["Dataset", "TestDefinition"],
}

This is useful information even if it's not actionable. We need to relax the "A test definition MUST refer to the test engine it is written for via conformsTo and to the engine’s version via engineVersion" requirement in the spec though.

[ilveroluca]

I can look into this for snakemake

[simleo]

We must make sure that LM is prepared for these changes before we start building these "new" crates.

I'm also wondering whether we need a "composite dataset" for the general case where tests for a workflow might consist of a mix of files and directories. In this case, though, I think we can avoid that complication by simply specifying that definition in TestSuite can point to multiple files / directories.