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

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

javatests/google/registry/rde/imports/XjcToDomainResourceConverterTest.java

@@ -426,7 +426,7 @@ public class XjcToDomainResourceConverterTest {
 
   private XjcRdeDomain loadDomainFromRdeXml(String filename) {
     try {
-      ByteSource source = RdeImportsTestData.get(filename);
+      ByteSource source = RdeImportsTestData.loadBytes(filename);
       try (InputStream ins = source.openStream()) {
         return ((XjcRdeDomainElement) unmarshaller.unmarshal(ins)).getValue();
       }