Fix race condition in get_free_port (#802)

[therohityadav]

Fixes #802

Problem

The get_free_port function was previously binding to "" (all interfaces). This caused a race condition where the OS would assign a port that appeared free on 0.0.0.0 but was actually unavailable or restricted when bitcoind specifically tried to bind to 127.0.0.1 milliseconds later.

Solution

I updated s.bind(("", 0)) to s.bind(("127.0.0.1", 0)).
This forces the OS to select a port specifically on the loopback interface ensuring it matches the interface bitcoind uses in the tests.

Verification

I verified locally that the function now returns a port explicitly bound to 127.0.0.1.