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/reporting/ActivityReportingQueryBuilderTest.java

@@ -61,7 +61,7 @@ public class ActivityReportingQueryBuilderTest {
       String actualTableName = String.format("%s_201709", queryName);
       String testFilename = String.format("%s_test.sql", queryName);
       assertThat(actualQueries.get(actualTableName))
-          .isEqualTo(ReportingTestData.getString(testFilename));
+          .isEqualTo(ReportingTestData.loadFile(testFilename));
     }
   }