[skara] RFR: Add additional sync label tests, adjust the implementation to match

[Robin Westberg]

Adjust the implementation of the sync label rules to match expectations.

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Commit messages:
 - Add additional sync label tests, adjust the implementation to match

Changes: https://git.openjdk.java.net/skara/pull/1033/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1033&range=00
  Stats: 148 lines in 3 files changed: 132 ins; 4 del; 12 mod
  Patch: https://git.openjdk.java.net/skara/pull/1033.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1033/head:pull/1033

PR: https://git.openjdk.java.net/skara/pull/1033