RFR: 8349475: Test tools/javac/api/TestJavacTaskWithWarning.java writes files in src dir

[Joe Darcy]

Correct generating files into the work directory, not the directory of test sources.

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Commit messages:
 - JDK-8349475: Test tools/javac/api/TestJavacTaskWithWarning.java writes files in src dir

Changes: https://git.openjdk.org/jdk/pull/23527/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23527&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349475
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23527.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23527/head:pull/23527

PR: https://git.openjdk.org/jdk/pull/23527