RFR: 8262049: [TESTBUG] Shenandoah: Adjustments in TestReferenceRefersTo.java for IU mode

[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

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

Commit messages:
 - 8262049: [TESTBUG] Shenandoah: Adjustments in TestReferenceRefersTo.java for IU mode

Changes: https://git.openjdk.java.net/jdk/pull/2653/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2653&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262049
  Stats: 18 lines in 1 file changed: 12 ins; 0 del; 6 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