RFR: 8367279: Test tools/javac/tree/TreePosTest.java timed out

[Jan Lahoda]

The `test/langtools/tools/javac/tree/TreePosTest.java` tries to parse all langtools tests, and started to time out recently in some cases. In the case of this test, we can try to use the `JavacTaskPool`, which pools and reuses javac instances, rather than creating (and initializing) a javac instance for each file. Given this test only parses the file, and even attributes them, this should be relatively safe.

Locally, I can't confirm too significant time improvements (before this change ~6s, after this change ~4s), but it might help on CI.

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Commit messages:
 - 8367279: Test tools/javac/tree/TreePosTest.java timed out

Changes: https://git.openjdk.org/jdk/pull/27334/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27334&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367279
  Stats: 29 lines in 1 file changed: 6 ins; 8 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/27334.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27334/head:pull/27334

PR: https://git.openjdk.org/jdk/pull/27334