Problem: MAX_SOCKET_STRING is too short for ipc wildcard binds

Solution: increase size to 256

tests/testutil.hpp

@@ -43,7 +43,9 @@
 //  settled. Tested to work reliably at 1 msec on a fast PC.
 #define SETTLE_TIME 300 //  In msec
 //  Commonly used buffer size for ZMQ_LAST_ENDPOINT
-#define MAX_SOCKET_STRING sizeof ("tcp://[::ffff:127.127.127.127]:65536")
+//  this used to be sizeof ("tcp://[::ffff:127.127.127.127]:65536"), but this
+//  may be too short for ipc wildcard binds, e.g.
+#define MAX_SOCKET_STRING 256
 
 //  We need to test codepaths with non-random bind ports. List them here to
 //  keep them unique, to allow parallel test runs.