Unfortunately InfoPath does not provide a test harness to allow for automating testing. This is something we've asked the InfoPath team for repeatedly. We're still hoping a rich runtime-om might surface to allow for this type of thing.
At this point having sample data files and previewing from designer with them frequently can help speed unit testing a lot. Also having a short unit test or build verification test script that you can run yourself manually makes it go faster.
Shiraz Cupala / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge