RFR: 8310143: RandomCommandsTest fails due to unexpected VM exit code after JDK-8282797

Mon Jun 19 11:37:10 UTC 2023

The fix for [JDK-8282797](https://bugs.openjdk.org/browse/JDK-8282797) missed that randomly generated Compile Commands can be invalid. I verified with the corresponding seeds that all occurrences of this intermittent failure are now fixed. I also removed `Executor::isValid` because it's unused.

Thanks,
Tobias

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Commit messages:
 - Screwed up the merge

Changes: https://git.openjdk.org/jdk/pull/14538/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14538&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310143
  Stats: 14 lines in 3 files changed: 3 ins; 4 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/14538.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14538/head:pull/14538

PR: https://git.openjdk.org/jdk/pull/14538