Move report-writing to the end.

I independently fixed the same issue as pull request #2, and then realized it was already merged in master. My fix is a bit more complete though; there's no reason to write out reports for every test file that will just be overwritten by the next one. This pull request avoids that; the coverage handler just adds coverage data to the collector, and all report-writing and status-collection happens once at the end of the run.