fix(tests): improve TOCTOU checks in getFreePort

[xnyo]

Improves TOCTOU checks in getFreePort to avoid flaky CI failures due to errors like this:

TestContext/pull_request/non-fork_from_regular_user_(testing): [] Using docker host 'unix:///var/run/docker.sock', and daemon socket 'unix:///var/run/docker.sock'
  TestContext/pull_request/non-fork_from_regular_user_(testing): [] Start server on http://172.16.253.54:44963/
  TestContext/pull_request/non-fork_from_regular_user_(testing): [] listen tcp 172.16.253.54:44963: bind: address already in use

Example: https://github.com/grafana/plugin-ci-workflows/actions/runs/26446584668/job/77854387243