Add tests for record encryption/decryption

This commit adds tests exercising mutually inverse pairs of
record encryption and decryption transformations for the various
transformation types allowed in TLS: Stream, CBC, and AEAD.
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 9f97501..79654c1 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -913,4 +913,14 @@
 }
 #endif
 
+void mbedtls_ssl_transform_init( mbedtls_ssl_transform *transform );
+int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
+                             mbedtls_ssl_transform *transform,
+                             mbedtls_record *rec,
+                             int (*f_rng)(void *, unsigned char *, size_t),
+                             void *p_rng );
+int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context *ssl,
+                             mbedtls_ssl_transform *transform,
+                             mbedtls_record *rec );
+
 #endif /* ssl_internal.h */