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

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

 {
   "actual-results": {
     "failed": null, 
     "failure-ignored": null, 
     "no-comparison": null, 
     "succeeded": null
   }, 
   "expected-results": {
     "3x3bitmaprect_565.png": {
       "allowed-digests": [
@@ skipping 107 matching lines @@
           16285974094717334658
         ]
       ], 
       "bugs": [
         1578
       ], 
       "reviewed-by-human": false
     }
   }
 }