gm: add self-test to confirm that ignore-failure field is respected

gm/tests/outputs/ignoring-some-failures/output-expected/json-summary.txt

 {
    "actual-results" : {
       "failed" : {
-         "565/selftest1.png" : [ "bitmap-64bitMD5", 12927999507540085554 ],
          "8888/selftest1.png" : [ "bitmap-64bitMD5", 1209453360120438698 ]
       },
-      "failure-ignored" : null,
+      "failure-ignored" : {
+         "565/selftest1.png" : [ "bitmap-64bitMD5", 12927999507540085554 ]

[epoger, 2013/08/30 19:53:02]

As expected... we are now ignoring failure for the test that was marked as
"ignore-failure":true ... but not the other one.

+      },
       "no-comparison" : null,
       "succeeded" : null
    },
    "expected-results" : {
       "565/selftest1.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 8863920166200910451 ]
          ],
-         "ignore-failure" : false
+         "ignore-failure" : true
       },
       "8888/selftest1.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 13451349865803053525 ]
          ],
          "ignore-failure" : false
       }
    }
 }