we already had a H2O_SANDBOX_NAME os env variable michal was using to avoid conflict on the sandbox dir.
I added H2O_PORT_OFFSET, which can have an integer value from 8 to 256 or not be set (equivalent to 0)
it adds the value to the existing base_port calculations everywhere (i.e. offsets whatever the test, plus jvm count, plus # of machines etc, calculates things to. Tests, if they specify a base_port, are 54321-54327. So gapping ports by 8 for single jvm, 3*8 (maybe 5*8) and more for multi-machine, should allow the tests to be run on the same machine without conflict. The testdir_single_jvm tests don't load a system much, so you would think we could have up to 4 lists of tests running on a machine and not slow down things too much (although it creates interesting variances in h2o behavior, since timing will get shifted around depending on what else the machine is doing..may cause intermittent cases that are hard to debug.