RFR: 2329: Close org.openjdk.skara.process.Execution

[Pavel Rappo]

Please review this trivial fix: org.openjdk.skara.process.Execution is AutoCloseable and a few tests don't close it.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/skara/pull/1678/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1678&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2329
  Stats: 32 lines in 2 files changed: 5 ins; 0 del; 27 mod
  Patch: https://git.openjdk.org/skara/pull/1678.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1678/head:pull/1678

PR: https://git.openjdk.org/skara/pull/1678