Integration tests: scan for open port
Integration tests: scan for open port

Previously our integration tests bound to port 53, and, if that failed,
fell back to binding to port 3553.

This commit introduces code to scan for an open port and uses that,
which lays the foundation for potentially parallelizing the integration
tests.

cunnie/sslip.io,cunnie/sslip.io,cunnie/sslip.io,cunnie/sslip.io