Create fewer predictable port clashes in management tests

Tests in test/jdk/javax/management/remote/mandatory frequently run together, and try to use the same port for their call to LocateRegistry.createRegistry(port);

The tests have a loop intended to avoid port clashes: this tries a very wide range of ports, and although it does detect a port that is in use, it rarely needs to retry even a few times.

Specifically on linux-aarch64 the port in use detection appears "racy" such that the clash sometimes goes unnoticed until it's too late, causing test failures.