Improve balancing of CI test jobs

Adding some related issue in this 'test work allocation optimization' space that have similar intent, ftr and later ease of reference.

I think this is good, probably we just need to try it and see its performance. The test coverage is also good to ensure isolation between the bins.

The only thing I would suggest here is to add a 'weight' column to the debug-discovery report, near to the 'count' column, so to make more visually clear the logic for the sorting, as this is no longer obvious.

Test execution sequence. Tests marked *** will be executed in this PARALLEL BIN #2.
-------------------------------------------------------------------------------------
    Sort  Bin                                 
Bin  Seq  Seq Slow? Group            Cnt Class
-------------------------------------------------------------------------------------
       1      #slow media             52 Drupal\Tests\media\Kernel\MediaAccessControlHandlerTest     
***    2    1 #slow package_manager   30 Drupal\Tests\package_manager\Kernel\SandboxManagerBaseTest  
       3      #slow Database          25 Drupal\Tests\mysql\Kernel\mysql\SchemaTest                  
       4      #slow Entity            22 …elTests\Core\Entity\EntityDefinitionUpdateMultipleTypesTest

Tweaking the weight of the #slow tests would probably make sense if we end up with major WT differences between bins of the same test type.

Let’s see first; time will tell. RTBC