Better behaved tests on "multi-cores"

[andrew_peterson]

Many of our tests repeat the same calls on 1 and 2 cores, presumably to make sure that it works in serial (worker) and parallel (master) mode. It is good to test this, but can be a little unexpected if the user has allocated only 1 core to the tests and they suddenly take over 2 cores.

Note that cores=1 means serial/worker operation, and cores={'localhost': 2} means parallel/master operation. So I think we could actually do cores={'localhost': 1} to still test master mode, but not occupy more than one core.