tls-crypto: Add support to configure DH groups to use

And add new test cases to test TLS 1.3 connections for each supported DH
group.

src/libtls/tls_crypto.h

@@ -669,6 +669,14 @@ tls_crypto_t *tls_crypto_create(tls_t *tls, tls_cache_t *cache);
 int tls_crypto_get_supported_suites(bool null, tls_version_t version,
 									tls_cipher_suite_t **suites);
 
+/**
+ * Get a list of all supported TLS DH groups.
+ *
+ * @param groups		pointer to allocated DH group array, to free(), or NULL
+ * @return				number of curves supported
+ */
+int tls_crypto_get_supported_groups(diffie_hellman_group_t **groups);
+
 /**
  * Get the TLS curve of a given EC DH group
  *