exp/services/recoverysigner: Fix TestOpen_openAndPingFails

[paulbellamy]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Extracted from #3545. I fixed TestOpen_openAndPingFails. The two issues were, Open on port 0, and sometimes the error would be different due to IPv6. Now it handles both ipv4 and ipv6, as well as finding a free port for the test to run on.

Why

Because it was intermittently failing and being annoying.

Known limitations

[TODO or N/A]

[leighmcculloch]

Thanks for fixing this! One question below, 👍🏻 regardless.