Put subsuites with a few tests in one group

[WeizhongX]

As we will need to restart the browser when starting a new subsuite, putting tests in one group can help reduce the number of restarts needed.

Keep an up limit of the size of such a group, so that we do not create a huge group that could potentially cause a worker to out live all other workers, thus increase the run time.

Bug: https://buganizer.corp.google.com/issues/341581718

[WeizhongX]

Having a command line argument sounds good to me.

[WeizhongX]

@jgraham, can you take another look at this? Thanks!