Fix silently skipped collection output assertions in workflow tests

[jmchilton]

Summary

Fixes #1705.

Two bugs in planemo/test/_check_output.py, both of which silently skip a collection output assertion:

• for_collections() matched only element_tests. Now also matches class: Collection, which is the only gate under which a collection assertion carrying no per-element tests — collection_type or element_count alone — is expressible. On master such an output matches nothing, falls through to the file checker, and fails with an unrelated "No path specified for expected output file".
• _check_output_collection() used TestCollectionOutputDef.from_dict(), which discards top-level collection_type, so the collection type was never asserted. Switched to from_yaml_test_format(), which maps it onto attributes["type"] and resolves the elements alias.

The same predicate gates planemo/runnable.py:402, so this fixes both call sites.

Deliberately not matching a bare elements: key, though Galaxy's framework-test gate does. It is documented in docs/test_format.rst only for collection inputs, no IWC workflow uses it for an output, and on its own it does not declare a collection — it stays an alias resolved once the output is known to be one. That leaves bare elements: as the single remaining spelling the two tools disagree on.

No dependency bump needed — from_yaml_test_format has been in galaxy-tool-util since 25.1, the floor of the current pin.

Testing

New tests/test_check_output.py, 9 pure unit tests, no Galaxy server. 5 fail against current master, including a list assertion against a list:paired collection reporting zero problems, and a class: Collection + element_count output that nothing detected at all. All 9 pass after. Black/isort/flake8 clean.

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