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/domain/DomainAllocateFlowTest.java

@@ -135,7 +135,7 @@ public class DomainAllocateFlowTest
     runFlowAssertResponse(
         CommitMode.LIVE,
         UserPrivileges.SUPERUSER,
-        readFile("domain_allocate_response.xml"));
+        loadFile("domain_allocate_response.xml"));
     // Check that the domain was created and persisted with a history entry.
     DomainResource domain = reloadResourceByForeignKey();
     assertAboutDomains().that(domain)