wptrunner does not continuously use the requested number of processes

[jdm]

I've watched it start with the desired number, then slowly trail off until a single process is in use at a time. This should be investigated, as it should significantly improve our test times if fixed.

cc @jgraham

[jdm]

Wild speculation: maybe the testes are divvied up at the start of the run, and if one set finishes before the others the remaining work is not redistributed.