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/CreateLrpTokensCommandTest.java

@@ -258,7 +258,7 @@ public class CreateLrpTokensCommandTest extends CommandTestCase<CreateLrpTokensC
 
   @Test
   public void testFailure_badTld() throws Exception {
-    thrown.expect(IllegalArgumentException.class, "TLD foo does not exist");
+    thrown.expect(IllegalArgumentException.class, "TLDs do not exist: foo");
     runCommand("--assignee=domain.tld", "--tlds=foo");
   }