Enable resume for test cases

[zhuchcn]

Would be very useful to be able to resume the test cases.

A quick thought is, we can save the .nextflow directory and .nextflow.log to test-case specific folder. We also save the mapping from test-case to nextflow cache directory to a temporary place, for example a '.nftest' directory. Then during the launching stage, we can look for whether the cache files for the specific test case exist.