[jdk17u] RFR: 8270171: Shenandoah: Cleanup TestStringDedup and TestStringDedupStress tests

[Aleksey Shipilev]

Clean backport to improve testing.

Additional testing: 
  - [x] Affected test still pass

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

Commit messages:
 - Backport e4c24f01ef97de1c9a5a5faca5dd8ea2ad8c62d9

Changes: https://git.openjdk.java.net/jdk17u/pull/58/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=58&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270171
  Stats: 20 lines in 2 files changed: 0 ins; 19 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/58.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/58/head:pull/58

PR: https://git.openjdk.java.net/jdk17u/pull/58