RFR: 8352316: More MergeStoreBench

[Shaojin Wen]

Added performance tests related to String.getBytes/String.getChars/StringBuilder.append/System.arraycopy in constant scenarios to verify whether MergeStore works

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Commit messages:
 - more MergeStoreBench

Changes: https://git.openjdk.org/jdk/pull/24108/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24108&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352316
  Stats: 116 lines in 1 file changed: 116 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24108.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24108/head:pull/24108

PR: https://git.openjdk.org/jdk/pull/24108