By clearing various difficulties of Classic, you can unlock different levels of Target Test as outlined below. Note that if you unlock a higher difficulty level before an lower one, you will unlock all the ones before it e.g. If you clear Classic mode once on Intense, you will unlock all the target tests.

Unlockable	How to Unlock
Level 2 (Medium)	Beat Target Test in Classic Mode twice on Easy or once on Medium
Level 3 (Hard)	Beat Target Test in Classic Mode twice on Medium or once on Hard
Level 4 (Very Hard)	Beat Target Test in Classic Mode twice on Hard or once on Very Hard
Level 5 (Intense)	Beat Target Test in Classic Mode twice on Very Hard or once on Intense