Handling invalid output object from engines

[tom-tan]

This request fixes #205 by handling JSONDecodeError thrown from json.loads when parsing an output object.
I also added a test for it.

Note that I extend tests.util.run_with_mock_cwl_runner to accept an executor as an optional argument.

After merging this request, an example in #205 is fixed as follows:

$ cwltest --tool tests/test-data/dummy-executor.sh --test tests/test-data/nothing.yml --junit-xml=result.xml
Test [1/2] do_nothing: Example of doing nothing
Test 1 failed: tests/test-data/dummy-executor.sh --outdir=/tmp/tmpsxaoak8e --quiet tests/test-data/true.cwl tests/test-data/empty.yml
Example of doing nothing
Output is not a valid JSON document: 'it is not JSON format!
'
Test [2/2] do_nothing2: Example of doing nothing more
Test 2 failed: tests/test-data/dummy-executor.sh --outdir=/tmp/tmpzsql2f61 --quiet tests/test-data/true.cwl tests/test-data/empty.yml
Example of doing nothing more
Output is not a valid JSON document: 'it is not JSON format!
'
0 tests passed, 2 failures, 0 unsupported features
$ ls result.xml 
result.xml

[mr-c]

Thank you @tom-tan !

[mr-c]

Thanks again @tom-tan ; next time can you create your branch here, and not on your personal fork? We are having issues with codecov.io and PRs made from forks :-(