test_result: Retry flaky tests

ERP5 functional tests are failing too often, until we improve the tests or the
code so that we don't have these intermitent failures, restart the failed tests
a bit automatically.

This is of course not good, because it's hiding problems, but the current state
of the test suite with many randomly failing test makes that we sometimes miss
when permanent failures are introduced.

See merge request nexedi/erp5!1206