[jdk11u-dev] RFR: 8186548: move jdk.testlibrary.JcmdBase closer to tests

[Goetz Lindenmaier]

I backport this to simplify follow up backports.

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Commit messages:
 - Backport 56ee0fccb33a3b542da89ff252d5f4e5aa6d0ddb

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/878/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=878&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8186548
  Stats: 10 lines in 3 files changed: 4 ins; 4 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/878.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/878/head:pull/878

PR: https://git.openjdk.java.net/jdk11u-dev/pull/878