RFR: 8364927: Add @requires annotation to TestReclaimStringsLeaksMemory.java

[jonghoonpark]

This change updates `TestReclaimStringsLeaksMemory.java` to properly guard against unsupported GC arguments.
Previously, the test only checked `@requires vm.gc == "null"`.

The fix separates the tests and adds explicit `@requires` clauses for each GC configuration to ensure the test only runs when supported.

This prevents test failures on VMs that do not support the specified GC.

(This is my first contribution to the JDK. Thank you for your review and guidance.)

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Commit messages:
 - 8364927: Add @requires annotation to TestReclaimStringsLeaksMemory.java

Changes: https://git.openjdk.org/jdk/pull/26766/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26766&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364927
  Stats: 30 lines in 1 file changed: 30 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26766.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26766/head:pull/26766

PR: https://git.openjdk.org/jdk/pull/26766