change checker tool's report file to 'results.json' and log file to 'log.txt'

[junjun-zhang]

Message from James Eddy:

I was able to download and validate your test submission for the 'pcawg-sanger-variant-caller' workflow, so we're almost ready to go! However, I need your help with what should (hopefully) be a small fix:

Please update your checker tool so that it outputs results.json and log.txt instead of checker_report.txt and checker_report.log. The script that automatically validates submissions for
all workflows expects these filenames. Also note that results.json should be a valid JSON file — e.g.

{"overall": true, "steps": {"workflow_log": true, "germline.sv": true, "sv.qc-json": true, "somatic.sv": true}}

(if you exclude the "steps" field, that's fine)