rebaseline_server: extract common unittest code, and rename tests/ -> testdata/

gm/rebaseline_server/testdata/inputs/gm-actuals/Test-Mac10.7-MacMini4.1-GeForce320M-x86_64-Debug/actual-results.json

 {
    "actual-results" : {
       "failed" : null,
       "failure-ignored" : {
          "displacement_565.png" : [ "bitmap-64bitMD5", 4569468668668628191 ],
          "displacement_8888.png" : [ "bitmap-64bitMD5", 11401048196735046263 ],
          "displacement_gpu.png" : [ "bitmap-64bitMD5", 5698561127291561694 ],
          "displacement_pdf-mac.png" : [ "bitmap-64bitMD5", 12901125495691049846 ],
          "displacement_pdf-poppler.png" : [ "bitmap-64bitMD5", 16285974094717334658 ]
       },
@@ skipping 68 matching lines @@
          "ignore-failure" : true
       },
       "displacement_pdf-poppler.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 4741823272437589681 ]
          ],
          "ignore-failure" : true
       }
    }
 }