Investigate why CategoryRoutedAliasUpdateProcessorTest is prone to OOM

After SOLR-13131 it was observed that this test had a higher than normal rate of failure. Investigation of the failures has shown that the primary problem is that it seems to hit OOM about 0.2% of the time making it one of the tests with the most failures, and has lead to the test being @BadApple annotated. We should investigate why this is so and improve the test or the related infrastructure as needed.