Reduce number of indices created in ClusterSettingsIT (opensearch-project#20681)

When the number indices created in this test was sufficiently high,
tests would frequently hit this failure at some point when creating a
file:

```
Caused by: java.nio.file.FileSystemException: /var/jenkins/workspace/gradle-check/search/server/build/testrun/internalClusterTest/temp/org.opensearch.cluster.settings.ClusterSettingsIT_129BF1D803EFDA59-001/tempDir-002/node_s0/nodes/0/_state/_a1_Asserting_0.doc: Too many open files
  at org.apache.lucene.tests.mockfile.HandleLimitFS.onOpen(HandleLimitFS.java:67)
  at org.apache.lucene.tests.mockfile.HandleTrackingFS.callOpenHook(HandleTrackingFS.java:82)
  at org.apache.lucene.tests.mockfile.HandleTrackingFS.newOutputStream(HandleTrackingFS.java:163)
  at java.****/java.nio.file.Files.newOutputStream(Files.java:215)
```

This commit changes the random range from [10, 50] to be a fixed value
of 3. This still gets the same coverage without unnecessarily increasing
test runtime and introducing flakiness around file handle limits.

Signed-off-by: Andrew Ross <andrross@amazon.com>

Author: andrross

server/src/internalClusterTest/java/org/opensearch/cluster/settings/ClusterSettingsIT.java

@@ -570,8 +570,7 @@ public void testUserMetadata() {
     }
 
     public void testWithMultipleIndexCreationAndVerifySettingRegisteredOnce() {
-        int randomInt = randomIntBetween(10, 50);
-        for (int i = 0; i < randomInt; i++) {
+        for (int i = 0; i < 3; i++) {
             String indexName = "test" + i;
             assertAcked(prepareCreate(indexName).setSettings(Settings.builder().put(IndexMetadata.SETTING_BLOCKS_METADATA, true)));
             assertBlocked(client().admin().indices().prepareGetSettings(indexName), IndexMetadata.INDEX_METADATA_BLOCK);