RFR: 1670: Make body handling consistent in all Issue implementations

[Zhao Song]

When fixing [SKARA-1658](https://bugs.openjdk.org/browse/SKARA-1658), I found the `TestIssue#body` didn't mimic `GitHubPullRequest#body` and` GitLabMergeRequest#body` properly. So this will block SKARA-1658.

The `body()` method in `TestIssue` always returns the latest body from the 'remote' instead of the cache. So it behaves different from GitHubPullRequest and GitLabMergeRequest.

In this patch, `TestIssue#body` will return the cached body like `GitHubPullRequest#body` and` GitLabMergeRequest#body`. Also updated related tests.

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Commit messages:
 - SKARA-1670

Changes: https://git.openjdk.org/skara/pull/1417/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1417&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1670
  Stats: 125 lines in 9 files changed: 0 ins; 0 del; 125 mod
  Patch: https://git.openjdk.org/skara/pull/1417.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1417/head:pull/1417

PR: https://git.openjdk.org/skara/pull/1417