Jack,
That assumes AL actually tests the software (I doubt it) rather than the results. The IRS does not test software, just the results, and the IRS is very clear that their test data does not cover all scenarios.
https://revenue.alabama.gov/incometax/software_vendors.cfm
You made me look...
AL first requires the software to have been IRS approved. We know now the IRS approval is not reliable. Since AL talks about their schemas, I suspect their testing is similar to the IRS. The vendor submits the RESULTS of what their software creates, and AL washes it through some sort of verification. Whether or not the vendor tests all possible scenarios, or whether or not AL requires certain scenarios to be tested is not clear from their main vendor page. Even it the IRS or a state had built some sort of test data for all possible scenarios (unlikely), how would the IRS or state actually know the data was not manually created? Don;'t for a minute think the vendor has to send their software to a tax agency and the tax agency has someone who can operate the software for testing purposes. The onus is always on the software vendor, and those using the software.
In other words, your only guarantee is your own review and maybe what the software vendor provides. There is absolutely no guarantee from the taxing agency that the software they show as approved will be accurate, or was even used to make the test data which garnered the approval. Remember, the taxing agency is going to do at little as possible in their testing, as they are only looking to make their job easier (not have to manually process data).