Fix InternalTTestTests

`testReduceRandom` was bumping up against the serialization that I added
in #54776. This makes it use random values that reduce in ways that
don't cause the randomized serialization to fail.

Author: nik9000

x-pack/plugin/analytics/src/test/java/org/elasticsearch/xpack/analytics/ttest/InternalTTestTests.java

@@ -51,7 +51,13 @@ private TTestState randomState() {
     }
 
     private TTestStats randomStats() {
-        return new TTestStats(randomNonNegativeLong(), randomDouble(), randomDouble());
+        /*
+         * Use a count significantly less than Long.MAX_VALUE so the reduce
+         * phase doesn't wrap to a negative number. If it *did* then we'd
+         * try to serialize a negative number with writeVLong which throws
+         * an assertion in tests.
+         */
+        return new TTestStats(randomLongBetween(0, Integer.MAX_VALUE), randomDouble(), randomDouble());
     }
 
     @Override