RFR: 8354985: Add unit tests for Executor class from jpackage test lib

[Alexey Semenyuk]

- Add tests covering command output handling in the Executor class
 - Fix revealed bugs
 - Add a couple of `Executor.of()` convenience methods

The test is implemented in a way that it will remain mostly unchanged when Executor supports disabling of stderr and stdout streams.

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Commit messages:
 - Add tests for Executor and fix revealed bugs.

Changes: https://git.openjdk.org/jdk/pull/24737/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24737&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354985
  Stats: 393 lines in 2 files changed: 380 ins; 6 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/24737.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24737/head:pull/24737

PR: https://git.openjdk.org/jdk/pull/24737