test_runner may be constrained by single-threaded performance for batch runs

[jcollins-g]

While working on https://dart-review.googlesource.com/c/sdk/+/249662 I discovered that the command line generation for new tests in batch mode is on a critical path. I was both able to speed up the existing analyzer command line generation slightly and avoid regressing. However, that signals to me that the processing of batch test data may actually fall behind our ability to execute them, or is very close to that barrier in some conditions.

Finding out if there are other optimizations that could be made to improve batch processing could greatly increase bot and interactive test running performance.