[lworld] RFR: 8327257: [lworld] Tests are needed to stress class preloading with concurrent class loadings

[Paul Hübner]

Hi all,

This change strengthens our Valhalla-specific classloading test suite and guards against regressions. An overview of the new changes is as follows:
* `BigClassTreeClassLoader`: a utility classloader that generates huge class trees on the fly. The idea is to generate a tree that is deep (lots of parent classes) and very wide (classes have lots of value classes as fields, which have value class fields, etc.). 
* `ConcurrentClassLoadingTest` strains the classloading and preloading by classloading many classes at the same time.
* `LoadableDescriptorsTest` ensures that the `LoadableDescriptors` attribute can handle unexpected data. This may need to be revisited if https://bugs.openjdk.org/browse/JDK-8366907 changes the semantics.
* `PreLoadCircularityTest` ensures that preloading happens even when there are cycles (via inheritance, fields, and both). 
* `PreLoadDoesNotInitTest` is a regression test that ensures preloading will not initialize a class.
* `PreLoadFailuresDoNotImpactApplicationTest` ensures that a flaky classloader during preloading will not cause crashes as long as the classloader can find the class when it is actually used.
* `ValueClassInheritanceTest` is a regression test that ensures one cannot have a value class inherit from something that's not an abstract value class.

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Commit messages:
 - Reindent.
 - More reviewer comments.
 - Incorporate feedback.
 - Move to JUnit and refactor.
 - Concurrent non-deadlocking huge tree test.
 - Iterate.
 - First draft.

Changes: https://git.openjdk.org/valhalla/pull/1552/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1552&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327257
  Stats: 892 lines in 11 files changed: 892 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1552.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1552/head:pull/1552

PR: https://git.openjdk.org/valhalla/pull/1552