Metric proposal: Calculate the simulator tracks' idle time

(Edited) Problem Statement: The numSims configuration indicates the maximum number of active simulators allowed. Imagine them as channels/tracks to accommodate the launched simulators. There may be times where the number of simulators is less than maximum, before/between simulator launches and towards the end when there are no test bundles to pick up while some simulators are still testing. This simulator track idle time is not being tracked.

Proposal: Assuming there are 'n' simulator tracks waiting to get stuff done right from when the Bluepill starts... it can measure and report the tracks' processing time that wasn't used.
This metric should include the overhead of upfront computations (formation, splitting and/or ordering of test bundles, etc.,) and all the fragmentation towards the end of test execution where one or more tracks are idle, having no more test bundles to take up, while others are still busy.

Benefits: This would help us measure and compare multiple approaches (test-count based packing vs test-execution-time based packing of test bundles as discussed in https://github.com/linkedin/bluepill/issues/336) and identify when performance regressions happen.

No files, tests, or entry points are named. Start by reading the related discussion in issue #336 and tracing simulator-track scheduling from startup through test-bundle completion; done means defining and reporting the proposed idle-time metric, including setup overhead and end-of-run fragmentation.