We enable data_suffix option when creating Coverage instances which
causes the output files to include the hostname, pid, and random id.
Before each run erase is called which clears all existing coverage data
files. And then at the end of the test run use the combine method which
merges the reports.
We collect coverage in the main process which collects data from
imports and also when running in single process mode. In the workers we
collect coverage in run_subsuite. This creates more stats files than
strictly required but I don't see a better place to save the stats when
stopping workers.
Note that this has the side effect of enabling parallel testing in
Travis CI.
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