Make shellcheck tests optional for local development

[Gyan-max]

Currently, contributors need to have shellcheck installed locally in order to run all tests with cargo test -p wdl-lint --all-features. This creates an extra dependency that not all contributors may have readily available on their systems, especially on Windows where shellcheck installation is less common.
When shellcheck is not installed, tests fail with errors that are not related to the changes being made, which can be confusing and act as a barrier to contribution.

Benefits

• Lower barrier to entry for new contributors
• Faster test runs for developers not working on shellcheck-related functionality
• Clearer test failures related to actual code changes
• CI still ensures all tests pass in the complete environment
• This change would improve developer experience while maintaining code quality standards.