RFR: 8262049: [TESTBUG] Fix TestReferenceRefersTo.java for Shenandoah IU mode [v2]

[Roman Kennke]

> Shenandoah's IU mode allows referents to be cleared even when accessed during concurrent marking. The test TestReferenceRefersTo.java needs to be adjusted to allow for that.
> 
> Test:
>  - [x] TestReferenceRefersTo.java + Shenandoah/IU
>  - [x] TestReferenceRefersTo.java + Shenandoah/SATB
>  - [x] TestReferenceRefersTo.java + G1

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Split TestReferenceRefersTo test in generic and non-Shenandoah parts

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2653/files
  - new: https://git.openjdk.java.net/jdk/pull/2653/files/b1fffb02..9746bc85

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2653&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2653&range=00-01

  Stats: 230 lines in 2 files changed: 206 ins; 20 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2653.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2653/head:pull/2653

PR: https://git.openjdk.java.net/jdk/pull/2653