[test] Add big integer self-tests

These test vectors are generated using Perl's Math::BigInt.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/tests.c

 REQUIRE_OBJECT ( aes_cbc_test );
 REQUIRE_OBJECT ( hmac_drbg_test );
 REQUIRE_OBJECT ( hash_df_test );
+REQUIRE_OBJECT ( bigint_test );