Here is better place, because they all digests.

- Use EVP API instead of direct calls to gost-engine;
- Remove MAC tests (they should go into test_digest);
- Expand `testcases' array to be more readable;
- Add some asserts of IV and block sizes.
- Remove noisy hex dumps if there is no errors;
- Note: no new cipers are added.

Better name to add more ciphers to test. Also, move it up in the tests
just after digests.

Test if copying `EVP_MD_CTX' is working good for GOST digests.

Also, use testcases table and add more ciphers.

* Tests that gost-engine correctly computes the public key from the
private key. (Twice -- Alice and Bob.)

* Tests that gost-engine correctly computes the derived shared key.
(Twice -- Alice and Bob, should be identical.)

Handle abnormal exit of s_client/s_server processes.

Fixes partially #230.