tests: accept alternative privileged port allocation failure

This registers an additional failure message on failed privileged port
allocation, equally funcionally valid but previously not handled and causing
the test to fail when run in the NixOS sandbox.

Differential Revision: https://phab.mercurial-scm.org/D11741
17 jobs for branch/stable in 49 minutes and 12 seconds (queued for 5 seconds)
Name Stage Failure
test-py3-rhg Tests
Skipped test-sparse-fsmonitor.t: skipped
Skipped test-sparse-revlog.t: missing artifact, run "/tmp/mercurial-ci/tests/artifacts/scripts/generate-churning-bundle.py"
Skipped test-ssh-repoerror.t: system supports running with rhg as 'hg'
Skipped test-verify-repo-operations.py: missing feature: allow slow tests (use --allow-slow-tests)
Failed test-basic.t: output changed
# Ran 922 tests, 63 skipped, 1 failed.
python hash seed: 3592149986
Cleaning up file based variables
ERROR: Job failed: exit code 1