RFR: 8267544: (test) rmi test NonLocalSkeleton fails if network has multiple adapters with the same address

[Roger Riggs]

Correct test code that creates a set of unique hostnames from an array of non-unique hostname.
Was using Set.of that throws if the elements are not unique.

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Commit messages:
 - Correct API to get list of unique hostnames

Changes: https://git.openjdk.java.net/jdk/pull/4151/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4151&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267544
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4151.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4151/head:pull/4151

PR: https://git.openjdk.java.net/jdk/pull/4151