Introduce version comparing functions

This zero-cost abstraction allows to change the internal encoding
of TLS/DTLS versions in the future.
diff --git a/tests/suites/test_suite_ssl.function b/tests/suites/test_suite_ssl.function
index b177779..268d56c 100644
--- a/tests/suites/test_suite_ssl.function
+++ b/tests/suites/test_suite_ssl.function
@@ -121,7 +121,7 @@
         CHK( mbedtls_md_setup( &t_in->md_ctx_enc,  md_info, 1 ) == 0 );
         CHK( mbedtls_md_setup( &t_in->md_ctx_dec,  md_info, 1 ) == 0 );
 
-        if( ver > MBEDTLS_SSL_MINOR_VERSION_0 )
+        if( mbedtls_ssl_ver_gt( ver, MBEDTLS_SSL_MINOR_VERSION_0 ) )
         {
             CHK( mbedtls_md_hmac_starts( &t_in->md_ctx_enc,
                                          md0, maclen ) == 0 );