add test categories: 1 mandatory category, one for each optional feature in CWL

[common-workflow-language]

add test categories: 1 mandatory category, one for each optional feature in CWL

[mr-c]

see also #5

[mr-c]

Any test that doesn't have anything in requirements is mandatory

[mr-c]

Implement this using a new per test field named tags containing a list of strings

[anton-khodak]

I would suggest tags field to be optional: if it is not present, the test is mandatory.
So, in the end, the conformance_test.yml will look like:

# mandatory
- job: v1.0/null-expression2-job.json
  tool: v1.0/null-expression2-tool.cwl
  output:
    output: 2
  doc: Testing the string 'null' does not trip up an Any without a default value.

# optional
- job: v1.0/count-lines6-job.json
  output:
    count_output: [32, 2]
  tool: v1.0/count-lines6-wf.cwl
  tags: ["ScatterFeatureRequirement", "MultipleInputFeatureRequirement"]

[anton-khodak]

If a runner fails an optional test, does it have to be counted to not-supported, instead of failed?

[mr-c]

@anton-khodak fail is fail unless the runner reports 'not supported'