RFR: 8366872: Wrong number of template arguments in test in test_rbtree.cpp

[Casper Norrbin]

After merging #26981, it did not consider the changes made in #27011, making the templates no longer match.

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Commit messages:
 - template fix

Changes: https://git.openjdk.org/jdk/pull/27089/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27089&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366872
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27089.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27089/head:pull/27089

PR: https://git.openjdk.org/jdk/pull/27089