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/HelloFlowTest.java

@@ -14,7 +14,6 @@
 
 package google.registry.flows.session;
 
-import static google.registry.testing.TestDataHelper.loadFileWithSubstitutions;
 import static org.joda.time.format.ISODateTimeFormat.dateTimeNoMillis;
 
 import com.google.common.collect.ImmutableMap;
@@ -29,9 +28,7 @@ public class HelloFlowTest extends FlowTestCase<HelloFlow> {
     setEppInput("hello.xml");
     assertTransactionalFlow(false);
     runFlowAssertResponse(
-        loadFileWithSubstitutions(
-            getClass(),
-            "greeting.xml",
-            ImmutableMap.of("DATE", clock.nowUtc().toString(dateTimeNoMillis()))));
+        loadFile(
+            "greeting.xml", ImmutableMap.of("DATE", clock.nowUtc().toString(dateTimeNoMillis()))));
   }
 }