Simpletest Concurrency should be constrained by available processors

Currently, a configurable option on the simpletest assessment plugin is 'DCI_Concurrency'. The issue is that this gets written into the derived build.yml file, which makes it difficult to re-run that build.yml exactly on another environment that might not have 40 cores to work with.

So, we need a reliable way to determine the number of available cores and not allow the concurrency setting to be above that, regardless of how its configured.