Clean up test "load resources" methods and classes

There is a big mix of different "load Resources" from different libraries
depending on where you were and what type of resource you want. Now there is a
clear hirarchy:

ResourceUtils:
  for use in actual (non-test) code, reads a file from a context directory

TestDataHelper (uses ResourceUtils):
  for use in tests, reads a file from a context directory + "/testdata". Also
  caches the resource so calling it multiple times with the same file will
  not read the file multiple times.

Library specific helpers (e.g. ToolsTestData) (uses TestDataHelper):
  for use in that library's tests only, reads from a specific testdata directory.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177027533

javatests/google/registry/xjc/XmlTestdataTest.java

@@ -15,11 +15,11 @@
 package google.registry.xjc;
 
 import static com.google.common.truth.Truth.assertThat;
-import static google.registry.util.ResourceUtils.readResourceUtf8;
 import static google.registry.xjc.XjcXmlTransformer.unmarshal;
 import static java.nio.charset.StandardCharsets.UTF_8;
 
 import google.registry.testing.ExceptionRule;
+import google.registry.testing.TestDataHelper;
 import google.registry.xjc.epp.XjcEpp;
 import google.registry.xjc.rde.XjcRdeDeposit;
 import java.io.ByteArrayInputStream;
@@ -44,7 +44,7 @@ public class XmlTestdataTest {
 
     private Example(String filename) {
       this.xmlStream = new ByteArrayInputStream(
-          readResourceUtf8(XmlTestdataTest.class, "testdata/" + filename).getBytes(UTF_8));
+          TestDataHelper.loadFile(XmlTestdataTest.class, filename).getBytes(UTF_8));
     }
   }