tests: add a couple tests

tests: add a couple tests

  - Add zero/one sanity check tests for ecmult

  - Add unit test for secp256k1_scalar_split_lambda_var

  - Typo fix in `ge_equals_ge`; was comparing b->y to itself, should
    have been comparing a->y to b->y

  - Normalize y-coordinate in `random_group_element_test`; this is
    needed to pass random group elements as the first argument to
    `ge_equals_ge`, which I will do in a future commit.