RFR: 2082: Use correct .jcheck/conf in AdditionalConfiguration.java

[Erik Duveblad]

Hi all,

please review this patch that ensures that we pass the correct `.jcheck/conf` to [AdditionalConfiguration.java](https://github.com/openjdk/skara/blob/master/bots/pr/src/main/java/org/openjdk/skara/bots/pr/AdditionalConfiguration.java). The `.jcheck/conf` should either come from the "overriding" `.jcheck/conf` (if the PR bot is configured to use that) or the pull request's target branch.

I have added that test that passes with this patch but fails without it.

Thanks,
Erik

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Commit messages:
 - 2082

Changes: https://git.openjdk.org/skara/pull/1578/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1578&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2082
  Stats: 78 lines in 5 files changed: 64 ins; 2 del; 12 mod
  Patch: https://git.openjdk.org/skara/pull/1578.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1578/head:pull/1578

PR: https://git.openjdk.org/skara/pull/1578