RFR: 8262122: [TESTBUG] Shenandoah-specific variant of TestReferenceRefersTo

[Roman Kennke]

Before JDK-8262049, the test TestReferenceRefersTo.java has been failing with I-U mode, because it asserted that weak references would not be cleared when accessed during mark. JDK-8262049 split up the test into a generic part that removed the offending test, and a non-Shenandoah part that contains the test.
I think it would be useful to add the full test with Shenandoah runners under gc/shenandoah to include it in hotspot_gc_shenandoah runs.

Test:
 - [x] TestReferenceRefersToShenandoah.java
 - [ ] hotspot_gc_shenandoah

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Commit messages:
 - 8262122: [TESTBUG] Shenandoah-specific variant of TestReferenceRefersTo

Changes: https://git.openjdk.java.net/jdk/pull/2674/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2674&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262122
  Stats: 325 lines in 1 file changed: 325 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2674.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2674/head:pull/2674

PR: https://git.openjdk.java.net/jdk/pull/2674