IBSimilarity causes test failure in TestSynonymQuery

### Description

`IBSimilarity` with `DistributionSPL` can produce negative scores for very large tf values, which `TestSynonymQuery.testBoosts()` uses. We don't have a good way to suppress individual Similarity implementations for tests at the moment, but I think in any case we should be looking to fix the root Similarity itself? Impacts and competitive iterators all assume that scores can't be negative, a contract which this Similarity breaks.

### Gradle command to reproduce

On `branch_10x`:
```
./gradlew :lucene:core:test --tests "org.apache.lucene.search.TestSynonymQuery.testBoosts" -Ptests.jvms=12 -Ptests.jvmargs= -Ptests.seed=EC3CC718CF767FE7 -Ptests.useSecurityManager=false -Ptests.gui=true
-Ptests.file.encoding=US-ASCII -Ptests.vectorsize=default -Ptests.forceintegervectors=false
```

Start by running the provided Gradle command for lucene:core and TestSynonymQuery.testBoosts(). Then inspect IBSimilarity with DistributionSPL and the scoring path used by the test. Done means the Similarity no longer produces negative scores for the large term-frequency values involved and the reproducing test passes.

Written by the indexing model from the issue text.