Test opcodes via external API instead of testing internal functions

[xgreenx]

• https://github.com/FuelLabs/fuel-vm/pull/754
• https://github.com/FuelLabs/fuel-vm/pull/779
• Contract loading: CALL, LDC, CCP
• Logging: LOG, LOGD
• Memory allocation: CFEI, ALOC

[MitchTurner]

I think we could improve the description of this item some. Since part of the work is already done.