tests: use ports from ephemeral ports range instead of hardcoded ports

[kmike]

Currently tests can fail or behave incorrectly when splash instance is running. It could be better to use temporary ports for splash server, mock server and proxy server.

>>> import socket
>>> s = socket.socket()
>>> s.bind(("", 0))
>>> s.getsockname()
('0.0.0.0', 54485)

For proxy server it may require more code because its port is in config that is stored in VCS; it may be OK not to fix this issue for proxy server.