springboot Protect against available port actually being unavailable

Hi,

this is an attempt to fix #19801. I remembered an issue in the past that tackled things in the same way (see #19355 ) that I applied to NettyRSocketServerFactoryTests. Hopefully that helps stabilizing the test-suite.

Cheers, Christoph

Comment From: dreis2211

Seems like @wilkinsona had the same idea. Closing 😉