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/crypto/key_exchange.h

@@ -153,6 +153,8 @@ struct key_exchange_t {
 	bool (*get_public_key)(key_exchange_t *this, chunk_t *value)
 		__attribute__((warn_unused_result));
 
+#ifdef TESTABLE_KE
+
 	/**
 	 * Set a seed used for the derivation of private key material.
 	 *
@@ -167,6 +169,8 @@ struct key_exchange_t {
 	bool (*set_seed)(key_exchange_t *this, chunk_t value, drbg_t *drbg)
 		__attribute__((warn_unused_result));
 
+#endif /* TESTABLE_KE */
+
 	/**
 	 * Get the key exchange method used.
 	 *