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/testing/TestDataHelper.java

@@ -55,7 +55,7 @@ public final class TestDataHelper {
    * Loads a text file from the "testdata" directory relative to the location of the specified
    * context class, and substitutes in values for placeholders of the form <code>%tagname%</code>.
    */
-  public static String loadFileWithSubstitutions(
+  public static String loadFile(
       Class<?> context, String filename, Map<String, String> substitutions) {
     String fileContents = loadFile(context, filename);
     for (Entry<String, String> entry : nullToEmpty(substitutions).entrySet()) {