Further Fine-tune Drone Unit Test Parallelization

[Hamms]

Turns out that the way tests get chunked up with 5 parallel processors causes a persistent error:

========= FAIL["test_summarize_with_numbered_units", "UnitGroupTest", 309.5503188769944]
 test_summarize_with_numbered_units#UnitGroupTest (309.55s)
        Expected: "Unit 1 - unit1-title"
          Actual: "యూనిట్ 1 - unit1-title"
        test/models/unit_group_test.rb:638:in `block in <class:UnitGroupTest>'
        test/testing/setup_all_and_teardown_all.rb:36:in `run'

Attempting 6 to see if that's any better

See here for more context

[Hamms]

Merging without approval to unblock pipeline