The parallel test runner uses multiple workers to distribute the
workload. These workers are assigned a worker ID using a globally
incremented variable, which determines what test database to connect
to. When the worker ID surpasses the test database IDs Django will
crash.
This reduce likelihood of crashing parallel tests because
ParallelTestSuite will no longer create more workers than TestCases.
It won't eliminate the problem completely though because there are
other circumstances in which new workers can be created which can then
be assigned an "illegal" worker ID.
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