fix config serialization tests

Author: talevy

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/rollup/v2/RollupV2Config.java

@@ -54,7 +54,6 @@ public class RollupV2Config implements NamedWriteable, ToXContentObject {
     private static final ConstructingObjectParser<RollupV2Config, String> PARSER;
     static {
         PARSER = new ConstructingObjectParser<>(NAME, false, (args, sourceIndex) -> {
-            // TODO(talevy): fix RollupV2Config to include the target index so that it can be completely read from XContent
             String rollupIndex = (String) args[0];
             GroupConfig groupConfig = (GroupConfig) args[1];
             @SuppressWarnings("unchecked")

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/rollup/v2/RollupV2ConfigTests.java

@@ -33,11 +33,10 @@ void setUpSourceIndex() {
 
     @Override
     protected RollupV2Config createTestInstance() {
-        return randomConfig(random());
+        return randomConfig(random(), sourceIndex);
     }
 
-    public static RollupV2Config randomConfig(Random random) {
-        final String sourceIndex = randomAlphaOfLength(10);
+    public static RollupV2Config randomConfig(Random random, String sourceIndex) {
         final String rollupIndex = "rollup-" + sourceIndex;
         final TimeValue timeout = random.nextBoolean() ? null : ConfigTestHelpers.randomTimeout(random);
         final GroupConfig groupConfig = ConfigTestHelpers.randomGroupConfig(random);