[TEST] Account for increase in ML C++ memory usage (#30675)

Recent changes to the ML C++ have resulted in higher memory usage,
so fewer "by" fields can be analyzed in a given amount of model
memory.

Author: droberts195

x-pack/qa/ml-native-tests/src/test/java/org/elasticsearch/xpack/ml/integration/AutodetectMemoryLimitIT.java

@@ -134,7 +134,7 @@ public void testTooManyByFields() throws Exception {
         assertThat(modelSizeStats.getModelBytes(), lessThan(36000000L));
         assertThat(modelSizeStats.getModelBytes(), greaterThan(30000000L));
         assertThat(modelSizeStats.getTotalByFieldCount(), lessThan(1900L));
-        assertThat(modelSizeStats.getTotalByFieldCount(), greaterThan(1600L));
+        assertThat(modelSizeStats.getTotalByFieldCount(), greaterThan(1500L));
         assertThat(modelSizeStats.getMemoryStatus(), equalTo(ModelSizeStats.MemoryStatus.HARD_LIMIT));
     }