RFR: 8371421: [AIX] new test ProcessCloseTest fails

[Roger Riggs]

Replace the test case for the child writing to stderr and then exit 1. 
`cat` on AIX writes the error message to stdout.
Use JavaChild instead of `cat` to have the same sequence of output, input, and exit status.

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Commit messages:
 - 8371421: [AIX] new test ProcessCloseTest fails

Changes: https://git.openjdk.org/jdk/pull/28185/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28185&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371421
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28185.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28185/head:pull/28185

PR: https://git.openjdk.org/jdk/pull/28185