construct_hkdf_params
params = construct_hkdf_params("sha256", "secret", 6, "salt", "label");
if (!TEST_ptr(params = construct_hkdf_params(
params = construct_hkdf_params("blah", "secret", 6, "salt", "label");
params = construct_hkdf_params("sha256", "secret", 6, "salt", "label");
params = construct_hkdf_params("sha256", "", 0, "salt", "label");
params = construct_hkdf_params("sha256", "1", 1, "salt", "label");
params = construct_hkdf_params("sha256", "secret", 6, "", "label");