RFR: 8295278: Add parallel class loading tests

[Coleen Phillimore]

I moved the ParallelLoad test to a subdirectory, and created a new one called CallLoadClass that calls loadClass directly and then waits on the ClassLoader object, while another thread calls forName() to go through the SystemDictionary resolution.

I also made dedicated tests out of the bugs reported in
[JDK-4699981](https://bugs.openjdk.org/browse/JDK-4699981) ClassCircularityError thrown without reason during class loading and
[JDK-4670071](https://bugs.openjdk.org/browse/JDK-4670071) java.lang.ClassLoader.loadClassInternal(String) is too restrictive

Tested with tier1 on Oracle platforms. These tests are run in tier1.

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Commit messages:
 - 8295278: Add parallel class loading tests

Changes: https://git.openjdk.org/jdk/pull/10587/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10587&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295278
  Stats: 737 lines in 23 files changed: 735 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10587.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10587/head:pull/10587

PR: https://git.openjdk.org/jdk/pull/10587