RFR: 8297936: Use reachabilityFence to manage liveness in ClassUnload tests

[Coleen Phillimore]

On Tue, 14 Feb 2023 09:02:09 GMT, Afshin Zafari <duke at openjdk.org> wrote:

> ### Description
> Instead of using static instances or using an object in print to keep it alive, `java.lang.ref.Reference.reachabilityFence(Object)` is used to keep an object alive and avoid class unloading.
> 
> ### Tests
> linux-x64-debug, macosx-aarch64-debug, mach5 tier1-5

I know -Xcomp shows the problem but I don't really want these tests to be run by default with -Xcomp.  Can you remove Xcomp?

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Changes requested by coleenp (Reviewer).

PR: https://git.openjdk.org/jdk/pull/12552