This commit changes the backend testing framework to run
in parallel mode which is same as --processes=4. If --coverage
is supplied, we enforce serial mode, --processes=1, because
coverage is not compatible with parallel mode at the moment.
This adds the option '--rerun' to the `test-backend` infrastructure.
It runs the tests that failed during the last 'test-backend' run. It
works by stailing failed test info at var/last_test_failure.json
Cleaned up by Umair Khan and Tim Abbott.
This adds an assertion, when `test-backend` is run with `--coverage`,
that we have 100% test coverage on a list of files that we expect to.
There's a whitelist/blacklist, managed in tools/test-backend.
Fixes#3363.