Add assertTldsExist(Iterable<String>) to check multiple TLDs at once

This is better than calling assertTldExists() inside a for loop because you can throw a single exception reporting all bad TLDs at once rather than only getting as far as the first failure.  And then it's also a one-liner instead of 3 lines.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152412876

javatests/google/registry/tools/server/ListDomainsActionTest.java

@@ -61,7 +61,7 @@ public class ListDomainsActionTest extends ListActionTestCase {
         null,
         null,
         null,
-        "^TLD %%%badtld%%% does not exist$");
+        "^TLDs do not exist: %%%badtld%%%$");
   }
 
   @Test