RFR: 7903809: Invalid source release N with --enable-preview

[Christian Stein]

Please review this change to prevent "invalid source release N with --enable-preview" errors for when an explicit compile task targets a different JDK feature release number then the JDK under test (running the compilation).

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Commit messages:
 - 7903809: Invalid source release N with --enable-preview

Changes: https://git.openjdk.org/jtreg/pull/226/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=226&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903809
  Stats: 30 lines in 2 files changed: 15 ins; 8 del; 7 mod
  Patch: https://git.openjdk.org/jtreg/pull/226.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/226/head:pull/226

PR: https://git.openjdk.org/jtreg/pull/226