How extensive tests should be is debatable. Striving for 100% code coverage does not necessarily mean a block of code has been effectively tested.

Consider testing:

• Any complex conditions
• Acceptance of different input or expected values; including complex parameter validation
• Exit conditions (especially raised errors or exceptions)
• When writing a unit test, resist the temptation to test other functions or commands. A unit test is not responsible for making sure every command it calls works. That comes later