Go to the test you want to assign and click on 'Test Settings.' Then, choose the class in which the student who needs to take the test is enrolled. This was the same in the past, but now there is an additional option at the bottom, 'Select students.'

When you check this option, the names of the students will appear on the right hand side, and you can select the student(s) who need(s) to take the test (either beforehand or afterward).

After selecting, click on the red button 'Hand out to...', and the test will only be assigned to the chosen student(s). You can then start the test for these student(s).