Improve BigInt<>Bytes conversion helpers test coverage & address edge case issues

[gabrocheleau]

Our BigInt<>Bytes conversion helpers have been updated in the past few weeks, notably because of verkle work that required us to handle little endianness conversion. We have also encountered issues related to those helpers that could have been easily avoided if we had been testing those helpers more thoroughly.

There are also outstanding questions regarding those helpers (taken from #3169 ):

• Should bigIntToBytes on BigInt(0) return the empty array? (Note: this is super breaking so we cannot do it, we could introduce a new method?)
• Should setLengthLeft throw if the expected length is lower than the current length? (It is unknown if we should trim left or right 🤔 )