pkcli.test set case names programmatically

[robnagler]

Avoids code like this:

def test_controls_monitor(fc):
    sim = fc.sr_sim_data("FODO with instruments")
    sim.models.controlSettings.operationMode = "DeviceServer"
    sim.models.controlSettings.readOnly = "1"
    _run_test(sim, "instrumentAnimation", "device_server_monitor.txt")

def test_controls_opt1(fc):
    sim = fc.sr_sim_data("FODO with instruments")
    sim.models.optimizerSettings.method = "nmead"
    _run_test(sim, "instrumentAnimation", "madx_nmead.txt")
[repeated 5 times]

[robnagler]

We need the ability to add case names based on, say, case_dirs or manually, e.g. sirepo importer_test doesn't use case_dirs.