test(compute): simplify and re-enable flaky ITPaginationTest test case (googleapis#13288)

To make the test more reliable, the assertions are simplified to verify
that all zone names are non-null and at least 2 pages have been
traversed by the iterator.

This test case has been quite flaky historically (see [past
comment](googleapis#9134 (comment))).
Iterating occasionally short-circuits at different places through the
list of zones (which Gemini hypothesizes may be aligned with regional
boundaries), and the DEFAULT_ZONE used in the test's current final
assertion appears towards the end of the alphabetically-sorted list.

Fixes googleapis#11759

Author: whowes

java-compute/google-cloud-compute/src/test/java/com/google/cloud/compute/v1/integration/ITPaginationTest.java

@@ -30,7 +30,6 @@
 import org.junit.AfterClass;
 import org.junit.Assert;
 import org.junit.BeforeClass;
-import org.junit.Ignore;
 import org.junit.Test;
 
 public class ITPaginationTest extends BaseTest {
@@ -112,25 +111,20 @@ public void testPaginationNextToken() {
         Lists.newArrayList(Iterables.transform(nextPageWithToken.getValues(), Zone::getName)));
   }
 
-  @Ignore(value = "https://github.com/googleapis/google-cloud-java/issues/11759")
   @Test
   public void testPaginationIterating() {
     ListZonesRequest listZonesRequest =
         ListZonesRequest.newBuilder().setProject(DEFAULT_PROJECT).setMaxResults(1).build();
     ZonesClient.ListPagedResponse response = zonesClient.list(listZonesRequest);
-    boolean presented = false;
     int count = 0;
     for (Zone element : response.iterateAll()) {
+      Assert.assertNotNull(element.getName());
       count++;
-      if (element.getName().equals(DEFAULT_ZONE)) {
-        presented = true;
+      if (count >= 2) {
+        break;
       }
     }
-    Assert.assertTrue(
-        String.format(
-            "Zone %s was not found for %s in zones list (size: %d).",
-            DEFAULT_ZONE, DEFAULT_PROJECT, count),
-        presented);
+    Assert.assertTrue("Expected iterator to traverse multiple pages", count >= 2);
   }
 
   @Test