RFR: 8251944: Add Shenandoah test config to compiler/gcbarriers/UnsafeIntrinsicsTest.java

[Aleksey Shipilev]

There used to be failures in Shenandoah CAS handling code like that were caught by this test. Those were fixed in JDK-8255401. This change turns the test into regression test for it.

Additional testing:
 - [x] Affected test on `x86_64` fastdebug, release
 - [x] Affected test on `x86_32` fastdebug
 - [x] Affected test on `aarch64` fastdebug

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Commit messages:
 - Mention 8255401 in @bug
 - Make test pass in release
 - 8251944: Add Shenandoah test config to compiler/gcbarriers/UnsafeIntrinsicsTest.java

Changes: https://git.openjdk.java.net/jdk/pull/1693/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1693&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251944
  Stats: 24 lines in 1 file changed: 22 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1693.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1693/head:pull/1693

PR: https://git.openjdk.java.net/jdk/pull/1693