Test min pool size of zero for scaling executor

This commit expands the configuration test for scaling executors to
include the case where the min pool size is set to zero.

core/src/test/java/org/elasticsearch/threadpool/ScalingThreadPoolTests.java

@@ -42,7 +42,7 @@ public class ScalingThreadPoolTests extends ESThreadPoolTestCase {
 
         final int min;
         if (randomBoolean()) {
-            min = randomIntBetween(1, 8);
+            min = randomIntBetween(0, 8);
             builder.put("threadpool." + threadPoolName + ".min", min);
         } else {
             min = "generic".equals(threadPoolName) ? 4 : 1; // the defaults