RFR: 8270110: Shenandoah: Add test for JDK-8269661

[Zhengyu Gu]

I would like to add a test for JDK-8269661. 

Currently, JDK-8269661 only affects Shenandoah GC, as it is the only GC supports object pinning and string deduplication.

Test:
- [x] new test passes with JDK-8269661, fails after backout JDK-8269661 fix.

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

Commit messages:
 - Disable compact string
 - Merge branch 'master' into string_critical_dedup
 - update test
 - v2
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/4740/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4740&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270110
  Stats: 208 lines in 3 files changed: 207 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4740.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4740/head:pull/4740

PR: https://git.openjdk.java.net/jdk/pull/4740