RFR: 8357822: C2: Multiple string optimization tests are no longer testing string concatenation optimizations

[Daniel Skantz]

This PR updates a few tests to reintroduce testing of string concatenation optimizations since a few bugs have recently been identified in this area.

Selection criteria: performed a text search on the test suite and identified tests for string concatenations or string optimizations that are not currently compiled with `-XDstringConcat=inline` and are not using StringBuilders explicitly.

Testing: T1-4.

Extra testing: ran the tests manually with `-XX:+OptimizeStringConcat` and verified that the tests are exercising string optimizations after the fix.

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Commit messages:
 - fix up tests

Changes: https://git.openjdk.org/jdk/pull/25610/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25610&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357822
  Stats: 98 lines in 7 files changed: 92 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/25610.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25610/head:pull/25610

PR: https://git.openjdk.org/jdk/pull/25610