I totally agree, however, when a computer adminsters a test, you cut out the
ESL instructor completely, which is why it's important to design the test well.
A simple acknowledgement that should you have gotten questions #X & #Y wrong, and use American English, you are actually correct for your usage.
By delineating and highlighting the differences, you not only teach proper grammatical usage in one dialect, you aid the student in learning to recognize when a different system is in use.
Wholeman