Test cases for addition, subtraction, multiplication, and modulo in the secp256k1 finite field

[weijiekoh]

This PR adds test cases for addition, subtraction, multiplication, and modulo in the secp256k1 finite field. The circuits which perform these operations are in test/circuits and the test cases are defined in secp256k1_Fp.test.ts. The test cases are written in the same pattern as the other test files like bigint.test.ts.

It also refactors the bigint_to_array, bigint_to_tuple, and get_strided_bigint functions into test/utils.ts to reduce code duplication.