rebaseline.py self-test: validate that unknown fields are maintained

tools/tests/rebaseline/input/json1/Test-Android-GalaxyNexus-SGX540-Arm7-Debug/expected-results.json

 {
    "expected-results" : {
       "3x3bitmaprect_565.png" : {
          "allowed-digests" : null,
          "ignore-failure" : false
       },
       "3x3bitmaprect_8888.png" : {
          "allowed-digests" : null,
          "ignore-failure" : false
       },
       "aaclip_gpu.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 11899819492385205974 ]
          ],
-         "ignore-failure" : false
+         "ignore-failure" : false,
+         "unknown-extra-field" : "make sure that rebaseline.py maintains this unknown field within a record whose allowed-digest IS NOT modified"
       },
       "aarectmodes_565.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 14760033689012826769 ]
          ],
          "ignore-failure" : false
       },
       "imageblur_565.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 17796243856503591523 ]
          ],
-         "ignore-failure" : false
+         "ignore-failure" : false,
+         "unknown-extra-field" : "make sure that rebaseline.py maintains this unknown field within a record whose allowed-digest IS modified"
       },
       "imageblur_8888.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 7426416989687670152 ]
          ],
          "ignore-failure" : false
       },
       "shadertext3_8888.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 10593797161686785561 ]
          ],
          "ignore-failure" : false
       },
       "xfermodeimagefilter_pdf.png" : {
          "allowed-digests" : null,
          "ignore-failure" : false
       },
       "xfermodes_pdf.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 9151974350149210736 ]
          ],
          "ignore-failure" : false
       }
    }
 }