Rename backing texture testing routines

Review URL: https://codereview.chromium.org/1230193006
diff --git a/src/gpu/GrTest.cpp b/src/gpu/GrTest.cpp
index b070d84..34d75a8 100644
--- a/src/gpu/GrTest.cpp
+++ b/src/gpu/GrTest.cpp
@@ -235,10 +235,12 @@
 
     void didRemoveGpuTraceMarker() override {}
 
-    GrBackendObject createBackendTexture(void* pixels, int w, int h,
-                                         GrPixelConfig config) const override { return 0; }
-    bool isBackendTexture(GrBackendObject id) const override { return false;  }
-    void deleteBackendTexture(GrBackendObject id) const override {}
+    GrBackendObject createTestingOnlyBackendTexture(void* pixels, int w, int h,
+                                                    GrPixelConfig config) const override {
+        return 0; 
+    }
+    bool isTestingOnlyBackendTexture(GrBackendObject id) const override { return false; }
+    void deleteTestingOnlyBackendTexture(GrBackendObject id) const override {}
 
     typedef GrGpu INHERITED;
 };