Rename all testdata loading files to conform to a single naming scheme

The scheme is:
- loadBytes: returns a ByteSource of the data
- loadFile: returns a string using UTF8 encoding, optionally applying
  substitutions

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177606406

javatests/google/registry/flows/contact/ContactCheckFlowTest.java

@@ -63,7 +63,7 @@ public class ContactCheckFlowTest
   @Test
   public void testXmlMatches() throws Exception {
     persistActiveContact("sah8013");
-    runFlowAssertResponse(readFile("contact_check_response.xml"));
+    runFlowAssertResponse(loadFile("contact_check_response.xml"));
   }
 
   @Test