RFR: 8366897: RBTreeTest.IntrusiveTest and RBTreeTest.CustomVerify tests fail on non-debug builds

[Casper Norrbin]

#26981 introduced the possibility to supply the red-black tree with a custom verifier, however the call to this verifier was inside an assert, meaning it wasn't called for all builds. This results in two gtests failing.

This fix moves the call outside the assert, and then checks the result instead.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/27097/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27097&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366897
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27097.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27097/head:pull/27097

PR: https://git.openjdk.org/jdk/pull/27097