add host entry for SUT in ATH

[jtnord]

downstream of jenkinsci/acceptance-test-harness#1331 adds a host entry to avoid DNS lookups using the sslip.io DNS service.

Testing done

Proposed changelog entries

• Entry 1: Issue, human-readable text
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Proposed upgrade guidelines

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1. The Jira issue, if it exists, is well-described.
2. The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
3. There is automated testing or an explanation as to why this change has no tests.
4. New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
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7. For dependency updates, there are links to external changelogs and, if possible, full differentials.
8. For new APIs and extension points, there is a link to at least one consumer.

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