[Kerberos] Find if port is available before using it for Kdc server

[bizybot]

If the randomly selected port was already in use the Kerberos
tests would fail. This commit adds check to see if the network
port is available and if not continue to find one for KDC server.
If it does not find port after 100 retries it throws an exception.

Closes #34261

[elasticmachine]

Pinging @elastic/es-security

[tvernum]

I don't quite follow how this happens and/or how this will fix it.

NetworkUtil.getServerPort() already uses ServerSocket to request an unused port number.
Is the problem that it's binding to the wrong local address, or that it doesn't distinguish between TCP and UDP?

I simply don't follow why this change is going to solve a problem if getServerPort isn't.

[bizybot]

Yes, you are right the NetworkUtil.getServerPort() already uses ServerSocket to request an unused port number. But does not distinguish between TCP and UDP and also I found that in my test it usually fails for the UDP (even when the port was bound getServerPort() used to return that port).

[tvernum]

It doesn't make a lot of sense to use getServerPort to allocate the port number and then re-test it, and then loop again if it fails.
If the logic in getServerPort is incomplete for our use case, then we shouldn't bother calling it.

[bizybot]

Instead of NetworkUtil#getServerPort() logic, the change now selects a random port, tests for availability and then bind to it. Thanks.