Feature request: handling known failure testing

[tom-tan]

Sometimes we need to test that the workflow engine fails its execution with certain inputs as expected.

For example, section 11 in user guide shows an example CWL file and a job file that will fail its execution.
Another example is the conformance test for workflow engines for CWL. The specification says that the execution should fail its execution in some cases.

It will be nice if cwltest can handle this kind of tests.

Related: Issue #37 in user_guide

[tom-tan]

Closed by #38!