Add configure option to disable testing key exchange methods

If this is used, the functionality to set a private key/value/seed for
key exchange methods is removed (including from the interface to avoid
accidentally forgetting to wrap implementations and uses of set_seed()).

The set_seed() method is assigned outside the INIT() macro to avoid
potentially undefined behavior (preprocessing directives in macro
arguments).

The test done by the crypto tester is a simple functionality test.

src/libstrongswan/plugins/gmp/gmp_diffie_hellman.c

@@ -135,6 +135,8 @@ METHOD(key_exchange_t, get_public_key, bool,
 	return TRUE;
 }
 
+#ifdef TESTABLE_KE
+
 METHOD(key_exchange_t, set_seed, bool,
 	private_gmp_diffie_hellman_t *this, chunk_t value, drbg_t *drbg)
 {
@@ -144,6 +146,8 @@ METHOD(key_exchange_t, set_seed, bool,
 	return TRUE;
 }
 
+#endif /* TESTABLE_KE */
+
 METHOD(key_exchange_t, get_shared_secret, bool,
 	private_gmp_diffie_hellman_t *this, chunk_t *secret)
 {
@@ -228,7 +232,6 @@ static gmp_diffie_hellman_t *create_generic(key_exchange_method_t group,
 				.get_shared_secret = _get_shared_secret,
 				.set_public_key = _set_public_key,
 				.get_public_key = _get_public_key,
-				.set_seed = _set_seed,
 				.get_method = _get_method,
 				.destroy = _destroy,
 			},
@@ -237,6 +240,10 @@ static gmp_diffie_hellman_t *create_generic(key_exchange_method_t group,
 		.p_len = p.len,
 	);
 
+#ifdef TESTABLE_KE
+	this->public.ke.set_seed = _set_seed;
+#endif
+
 	mpz_init(this->p);
 	mpz_init(this->yb);
 	mpz_init(this->ya);