Problems with test-taking varied, but common issues were getting distracted by extraneous stimuli in "regular" classrooms. Frequently ADHD students were slow readers and really did need/use extra time. But the biggest problem was test-taking anxiety, often quite prominent, contributing to difficulty with tests.
Interestingly, more than not ADHD students were reluctant to ask for accommodations. Some never availed themselves of this benefit. Others only used part of the options they could use.
In my practice were a number of ADHD adults admitted to nursing school, some having serious academic difficulties and eventually asking for accommodations. The assistance was effective. Notably, none of these student-patients failed or dropped out, all graduated. A pleasant footnote to add is a subset of these nurses developed exceptional skill earning them leadership positions in their institutions.
Reputable clinicians wouldn't give an ADHD diagnosis or support academic accommodations without sufficient clinical evidence.