Fix potential deadlock on parallel test execution again
Follow up on c9799ad, see its message for details
This commits invert the worker lease holder test processor and the
max n parallel test processor, putting the former before the later.
Given that the “max n parallel test” processor uses non-blocking proxy
actors, a deadlock could happen when forkEvery=1.
This was discovered while working on ParallelTestExecutionIntegrationTest
flakiness.