Support running a single WDL task instead of a whole workflow

[unito-bot]

Apparently the OpenWDL tests are mostly built around running single tasks, but toil-wdl-runner can only run whole workflows. We should support running tasks.

This is easy when there’s just one task in a file. If you want to run one task defined in-line in a file with a workflow and multiple other tasks, we might need a way to let the user indicate that.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1578