Test test_bit_length_create_commitment_with_user_defined_randomness sometimes fails

[amanusk]

Related to #54, it seems that the test that fails once in 16 runs (in expectation) is:
https://github.com/KZen-networks/curv/blob/7f0586fd543f81f5202da766cd7c382e07876a04/src/cryptographic_primitives/commitments/hash_commitment.rs#L78
Because leading zeros are not translated into a string.

Possible fix suggestions:

• Check not_zero
• Check no error is returned from string conversion
• Check length > (some number less than 64) (This is will still be probabilistic but with lower probability)
• Ignore test

[omershlo]

we should find a better way to test probabilistic commitments