Fix integration tests on macOS

[thiagosalles]

Fix integration tests by binding the listener to localhost, just like the client

The integration tests were failing because the server listener was configured with net.Listen("tcp", ":8080"), without specifying a hostname, which binds to all network interfaces (including IPv6). Meanwhile, the client connection used net.Dial("tcp", "localhost:8080"), which resolved to IPv4 (127.0.0.1) on my machine.

The discrepancy between the listener accepting both IPv4 and IPv6 and the client specifically using IPv4 may have caused the X-Forwarded-For header to be populated with the client’s IP (127.0.0.1) instead of being empty, as the tests expected.

Changing the listener to net.Listen("tcp", "localhost:8080") ensures the server binds to the same interface as the client, aligning with the test expectations and fixing the failures.

Evidence

After the modification, the integration tests pass successfully:

$ go test -tags integration ./test
ok  	github.com/luraproject/lura/v2/test	3.107s

Fixes #753