Add known failure testing capability

[tom-tan]

This request is to solve #37 by adding known failure capability to cwltest.

To use this feature, just add should_fail: true to tests in which the execution should fail.
This field is optional. When you do not specify should_fail field, cwltest treats that should_fail is false.
For example:

- doc: Test for section 11 (1st example)
  job: 11-records/record-job1.yml
  tool: 11-records/record.cwl
  should_fail: true

When should_fail is false, cwltest behaves as same as before. That is, it passes a test when the execution succeeds and it fails a test when the execution fails.
When should_fail is true, cwltest fails a test when the execution succeeds.
Here is an example message for it (I added # comments for explanation):

Test failed: cwl-runner --outdir=/var/folders/gk/zyzswjbs6m59ywz69zhrtmlh0000gn/T/tmp7dsw3jn4 --quiet 04-output/tar.cwl 04-output/tar-job.yml
Test for section 4 # corresponding `doc` field in YAML file
Returned zero but it should be non-zero # error message

Note: we may need other fields for more precise checking or there may be a better field name than should_fail.

[mr-c]

👍 Can we get a test of this functionality?

[tom-tan]

I checked this request works by using CWL/user_guide repository.

Please save the following as _include/cwl/test1.yml in user_guide repository and run cwltest --test test1.yml in _include/cwl directory.

- doc: Test for section 4. This test will fail.
  job: 04-output/tar-job.yml
  tool: 04-output/tar.cwl
  should_fail: true

I used another test as follows. Please save it as _include/cwl/test2.yml in user_guide and run cwltest --test test2.yml in _include/cwl.

- doc: Test for section 11 (1st example). This test will succeed.
  job: 11-records/record-job1.yml
  tool: 11-records/record.cwl
  should_fail: true

[mr-c]

Thanks @tom-tan! We can add that to this repository as part of another PR.

[tom-tan]

Thanks!