add dedicated test to assert internals of LZ4 hashtable [LUCENE-9190]

This assert was called all the time by all tests, causing a bottleneck. I disabled it in #10227, but it would be nice to add a subclass or package-private method or something to still test it (without taking up tons of cpu).

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Migrated from [LUCENE-9190](https://issues.apache.org/jira/browse/LUCENE-9190) by Robert Muir (@rmuir)

Start by reviewing the LZ4 hashtable assertion and the change referenced in #10227, then inspect the existing tests that exercise it. Add a dedicated test path that preserves coverage of the assertion without running it for every test, and confirm the broad test suite no longer incurs the CPU bottleneck.

Written by the indexing model from the issue text.