Test cases for utils.toTwosComplement

[danieljbruce]

These are some test cases I created for the function toTwosComplement from web3.utils that didn't have any testing for this function. Next I plan on tackling something more closely related to a current codebase issue for which this is a precursor.

[coveralls]

Coverage remained the same at 90.557% when pulling e7721ff on danieljbruce:develop into 917d152 on ethereum:develop.

[coveralls]

Coverage remained the same at 90.557% when pulling e7721ff on danieljbruce:develop into 917d152 on ethereum:develop.

[frozeman]

Thanks for that, i will add those as well for the 1.0 branch

[frozeman]

I 1.0 it will work as follows:

    web3.utils.toTwosComplement('-1');
    > "0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"

    web3.utils.toTwosComplement(-1);
    > "0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"

    web3.utils.toTwosComplement('0x1');
    > "0x0000000000000000000000000000000000000000000000000000000000000001"

    web3.utils.toTwosComplement(-15);
    > "0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff1"

    web3.utils.toTwosComplement('-0x1');
    > "0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"