RFR: 8288989: Make tests not to depend on the source code

[Adam Sotona]

Two langtool tests depend on the JDK source tree presence. 
This patch skips the tests execution in a source-less test environment.

Please review.

Thanks,
Adam

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Commit messages:
 - 8288989: Make tests not to depend on the source code

Changes: https://git.openjdk.org/jdk/pull/16470/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16470&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288989
  Stats: 11 lines in 2 files changed: 8 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16470.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16470/head:pull/16470

PR: https://git.openjdk.org/jdk/pull/16470