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/flows/session/LogoutFlowTest.java

@@ -37,7 +37,7 @@ public class LogoutFlowTest extends FlowTestCase<LogoutFlow> {
   public void testSuccess() throws Exception {
     assertTransactionalFlow(false);
     // All flow tests are implicitly logged in, so logout should work.
-    runFlowAssertResponse(readFile("logout_response.xml"));
+    runFlowAssertResponse(loadFile("logout_response.xml"));
   }
 
   @Test