RFR: 8284556: Ensure reachability of classes in runtime/whitebox/TestHiddenClassIsAlive.java and serviceability/dcmd/vm/ClassLoaderHierarchyTest.java

[Leonid Mesnik]

Tests are updated to ensure that classes are alive while test checks them.
Actually, fixed by @AlanBateman in repo-loom.

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Commit messages:
 - 8284556: Ensure reachability of classes in runtime/whitebox/TestHiddenClassIsAlive.java and serviceability/dcmd/vm/ClassLoaderHierarchyTest.java

Changes: https://git.openjdk.java.net/jdk/pull/8152/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8152&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284556
  Stats: 11 lines in 2 files changed: 7 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8152.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8152/head:pull/8152

PR: https://git.openjdk.java.net/jdk/pull/8152