v23/syncbase: Add extra flags to syncbase ping pong benchmark
v23/syncbase: Add extra flags to syncbase ping pong benchmark

You can now add numSync and numGroup flags to the test runner in order
to vary the number of peers in the syncgroup as well as the number of
overall syncgroups.

This is going to be used to examine how latency changes as these
numbers vary.

For example, you can run with 2 syncgroups and 3 peers:
jiri go test -test.bench=BenchmarkPingPongPair -test.benchtime=5s
  -run XXX -numGroup=2 -numSync=3 -v23test-syncbase-debug-args "-vpath=*=0"

Change-Id: I23922f1740d7101538597acabd35c0c35cd81715

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23