Move the ability to load the dasync engine into ssltestlib.c

The sslapitest has a helper function to load the dasync engine which is
useful for testing pipelining. We would like to have the same facility
from sslbuffertest, so we move the function to the common location
ssltestlib.c

Follow on from CVE-2024-4741

test/ssltestlib.c

@@ -8,7 +8,7 @@
  */
 
 #include <string.h>
-
+#include <openssl/engine.h>
 #include "internal/nelem.h"
 #include "ssltestlib.h"
 #include "testutil.h"
@@ -947,3 +947,27 @@ void shutdown_ssl_connection(SSL *serverssl, SSL *clientssl)
     SSL_free(serverssl);
     SSL_free(clientssl);
 }
+
+ENGINE *load_dasync(void)
+{
+#if !defined(OPENSSL_NO_TLS1_2) && !defined(OPENSSL_NO_DYNAMIC_ENGINE)
+    ENGINE *e;
+
+    if (!TEST_ptr(e = ENGINE_by_id("dasync")))
+        return NULL;
+
+    if (!TEST_true(ENGINE_init(e))) {
+        ENGINE_free(e);
+        return NULL;
+    }
+
+    if (!TEST_true(ENGINE_register_ciphers(e))) {
+        ENGINE_free(e);
+        return NULL;
+    }
+
+    return e;
+#else
+    return NULL;
+#endif
+}

test/ssltestlib.h

@@ -52,5 +52,5 @@ int mempacket_test_inject(BIO *bio, const char *in, int inl, int pktnum,
 typedef struct mempacket_st MEMPACKET;
 
 DEFINE_STACK_OF(MEMPACKET)
-
+ENGINE *load_dasync(void);
 #endif /* OSSL_TEST_SSLTESTLIB_H */