Fix network.construction_with_specified_port (uses test network port)

[dsiganos]

The test case network.construction_with_specified_port uses the test
network port as the test port but that means this test fails if there
is a test network node running on that machine.

Convert the test to use nano::get_available_port() to avoid that
problem.

[theohax]

nano::get_available_port() is about to be gotten rid of completely, so perhaps use another static fixed port number?

[dsiganos]

Why are we removing nano::get_available_port? It seems very useful.
What will it be replaced by?

[clemahieu]

I don't think we should totally get rid of get_available_port, though most usages should be replaced where we don't need to specify a port; this case would be an exception I think where we do want to specify a port.

[theohax]

I don't think we should totally get rid of get_available_port, though most usages should be replaced where we don't need to specify a port; this case would be an exception I think where we do want to specify a port.

This might just leave room for bad usages of it IMHO, but I can see your point. Well so far it's being used everywhere nonetheless so I guess another one usage won't hurt too much.