RFR: 1633: PullRequestPollerTests#queryPaddingComment fails

[Erik Joelsson]

A combination of changes in SKARA-1584 and SKARA-1606 is causing a test failure. The problem is in `TestPullRequest::review`, where we will sometimes (most often) return the internal ArrayList<Review> from the underlying store object. This is causing TestPullRequest::snapshot to not return a pure snapshot, but instead something that gets updated when the store is updated, defeating the whole idea of the snapshot.

The fix is to wrap the return value with a `List::copyOf`. I'm also applying this to `Issue::comments` as that's also part of the snapshot and using unmodifiable collections is usually preferable.

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Commit messages:
 - SKARA-1633

Changes: https://git.openjdk.org/skara/pull/1394/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1394&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1633
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1394.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1394/head:pull/1394

PR: https://git.openjdk.org/skara/pull/1394