Allow two passes through test_cmd in simult_aio.

Allow two passes through test_cmd in simult_aio.

Fixed failing commands when the exact ns sample period was not available.

e.g:

Testing command:
Command on <pycomedi.common.PyComediIO object at 0x8e9184c> (output):
subdevice:      1
flags:          0x40
start:          ext|    18
scan_begin:     timer|  324675
convert:        now|    0
scan_end:       count|  1
stop:           count|  16010
test pass 0, argument conflict
Failing command (4):
Command on <pycomedi.common.PyComediIO object at 0x8e9184c> (output):
subdevice:      1
flags:          0x40
start:          ext|    18
scan_begin:     timer|  324700
convert:        now|    0
scan_end:       count|  1
stop:           count|  16010