EsIntegTestCase#setRandomIndexSettings mixes usage of the threadlocal random and supplied random

[jpountz]

This method is currently implemented this way:

    protected Settings.Builder setRandomIndexSettings(Random random, Settings.Builder builder) {
        setRandomIndexMergeSettings(random, builder);
        setRandomIndexTranslogSettings(random, builder);

        if (random.nextBoolean()) {
            builder.put(MergeSchedulerConfig.AUTO_THROTTLE_SETTING.getKey(), false);
        }

        if (random.nextBoolean()) {
            builder.put(IndicesRequestCache.INDEX_CACHE_REQUEST_ENABLED_SETTING.getKey(), random.nextBoolean());
        }

        if (random.nextBoolean()) {
            builder.put(IndexSettings.INDEX_CHECK_ON_STARTUP.getKey(), randomFrom("false", "checksum", "true")); // <-- uses threadlocal Random
        }

        if (randomBoolean()) {  // <- uses threadlocal Random
            // keep this low so we don't stall tests
            builder.put(UnassignedInfo.INDEX_DELAYED_NODE_LEFT_TIMEOUT_SETTING.getKey(),
                    RandomNumbers.randomIntBetween(random, 1, 15) + "ms");
        }

        return builder;
    }

It looks buggy as it is sometimes using the supplied Random and sometimes using the thread-local Random. I don't think it is a source of problems since it looks like the supplied Random is always equal to the thread-local Random, but we should clean this up?

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Build)