Require workflow tests under the IWC lint profile

[jmchilton]

Fixes #1693.

Summary

workflow_lint --iwc --fail_level error currently succeeds when Planemo cannot discover any tests for a workflow because the missing-test diagnostic is only a warning. This is weaker than the other IWC repository checks and allows an untested workflow to pass the IWC profile.

This change makes Workflow missing test cases. an error when the IWC profile is active. Outside --iwc, the diagnostic remains a warning and the existing default behavior is unchanged.

Tests

The fail-level test now verifies all three relevant outcomes:

• missing tests fail at the default warning fail level;
• missing tests still pass at --fail_level error outside the IWC profile; and
• missing tests fail as an error under --iwc --fail_level error.

pytest -q tests/test_cmd_workflow_lint.py
22 passed