RFR: 8294899: Process.waitFor() throws IllegalThreadStateException when a process on Windows returns an exit code of 259

[Roger Riggs]

Process.waitFor() throws IllegalThreadStateException when a process returns an exit code of 259.
As described in the bug report, `waitFor()` should be sensitive to the exit value.
Previously, it erroneously threw IllegalStateException.
Added a test to verify.

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Commit messages:
 - 8294899: Process.waitFor() throws IllegalThreadStateException

Changes: https://git.openjdk.org/jdk/pull/10680/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10680&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294899
  Stats: 50 lines in 2 files changed: 49 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10680.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10680/head:pull/10680

PR: https://git.openjdk.org/jdk/pull/10680