Some things I do:

1. Start the class working on a simple song together. Then when testing, they practice that new song.
2. Assign them partners. They practice together and tutor each other while you're testing.
3. Give them worksheets on note naming (lines and spaces) and have them work those while you test, or use other worksheets relating to music class stuff.
4. Have them play Skittle Staff (search the Idea Bank for the directions) or another note-naming game.

Something I've done for testing which works well is having laminated cards with numbers on them. The students who want to test are given a card and can only come up when I call their number. This helps prevent the line around my desk and across the room when they ALL want to test at the same time.