[lworld] RFR: 8281269: Update Valhalla test which uses ClassTransformer class

[Alex Menkov]

prerequisite for JDK-8281268

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Commit messages:
 - switch to ClassTransformer in test.lib

Changes: https://git.openjdk.java.net/valhalla/pull/687/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=687&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281269
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/687.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/687/head:pull/687

PR: https://git.openjdk.java.net/valhalla/pull/687