RFR: 1603: Make labels handling consistent in all Issue implementations

[Zhao Song]

Make TestIssue and JiraIssue update the cache when updating labels.

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Commit messages:
 - Make TestIssue and JiraIssue update the cache when updating labels

Changes: https://git.openjdk.org/skara/pull/1387/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1387&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1603
  Stats: 23 lines in 2 files changed: 17 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/skara/pull/1387.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1387/head:pull/1387

PR: https://git.openjdk.org/skara/pull/1387