CoverageTestWrapper masks 'parallel' option in rcfile
The runners.CoverageTestWrapper.start method always sets the data_suffix parameter to the coverage class, even if it's not set among CoverageTestWrapper's options (i.e., the option value is None). The result is that any value set for this option in coverage's rcfile is masked. See attached patch for fix. I have verified that the fix doesn't break anything by running the testsuite.