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

gm/rebaseline_server/testdata/inputs/gm-actuals/Test-Android-GalaxyNexus-SGX540-Arm7-Release/actual-results.json

 {
    "actual-results" : {
       "failed" : {
          "texdata_gpu.png" : [ "bitmap-64bitMD5", 3695033638604474475 ]
       },
       "failure-ignored" : {
          "filterbitmap_checkerboard_192_192_565.png" : [ "bitmap-64bitMD5", 4719210487426381700 ],
          "filterbitmap_checkerboard_192_192_8888.png" : [ "bitmap-64bitMD5", 3154864687054945306 ],
          "filterbitmap_checkerboard_32_2_565.png" : [ "bitmap-64bitMD5", 15528304435129737588 ],
          "filterbitmap_checkerboard_32_2_8888.png" : [ "bitmap-64bitMD5", 712827739969462165 ]
@@ skipping 77 matching lines @@
          "ignore-failure" : true
       },
       "texdata_gpu.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 2736593828543197285 ]
          ],
          "ignore-failure" : false
       }
    }
 }