Use echo instead of cat for number that isn't a file path

[stxue1]

Closes #654

~{number * 2} outputs a number, but cat ~{number *2} tries to read the file named that number, which doesn't work as that file does not exist. This test isn't testing reading from a file but from stdout, so echo should be more appropriate in printing the number to stdout.

I didn't add anything to the changelog as this is more of a bug fix.

Checklist

• Pull request details were added to CHANGELOG.md
• Valid examples WDL's were added or updated to the SPEC.md (see the guide on writing markdown tests)

[jdidion]

Rebasing to wdl-1.1.3