Add a method to create a server subprocesses only on unused local ports

[MVrachev]

Description of issue or feature request:
It will be useful to make sure we are starting the server subprocesses only on unused local ports.
This will save us issues when the server is started on used local ports and given that more tests using server subprocesses could be added in the future, then we have even a better stimulus to create such a method.

It's preferable if we find a way to do this atomically e.g. check if the port is busy and start the server almost instantly because we want to minimize the possibility that that port could be taken between the port validation and server startup.

Current behavior:
There is no such functionality in TUF and sometimes I am getting [Errno 98] Address already in use or ConnectionRefusedError: [Errno 111].

Expected behavior:
Hopefully, none of the above errors will ever appear after we add this functionality.

[MVrachev]

It seems that this issue is connected with: #1111