Use SierraGenerator for testing

[gabrielbosio]

Replacing Cairo code with Sierra in tests may have the following advantages:

• Reduces test execution time by removing the Cairo to Sierra compilation step.
• By using Sierra code instead of Cairo in the libfunc tests we ensure those tests are covering the expected libfuncs and we are not forced to change the tested programs because future changes that may happen in Cairo to Sierra compilation that could change what libfuncs a specific Cairo program may use.