As noted in T50217#10270643, when there are failures in PHPUnit parallel test runs, it is harder now to find the test failures in the log output because test failures are followed by successful runs from other split groups in the sequential log output. In particular, the You should really speed up these slow tests... messages are more annoying now because they make it hard to quickly scan the log output to find the errors.

Buffer the output of the split_groups and collect information about failed groups, presenting the details of failed tests in one block at the end of the jobs' console log.

(see this code in the EarlyWarning bot for a possible approach to parsing / capturing PHPUnit output)