GM: Allow ignored-tests.txt to list configs as well as tests (and combinations too)

gm/tests/outputs/ignoring-one-test/output-expected/json-summary.txt

 {
    "actual-results" : {
-      "failed" : null,
+      "failed" : {
+         "565/selftest1.png" : [ "bitmap-64bitMD5", 12927999507540085554 ]
+      },
       "failure-ignored" : {
-         "565/selftest1.png" : [ "bitmap-64bitMD5", 12927999507540085554 ],
          "8888/selftest1.png" : [ "bitmap-64bitMD5", 1209453360120438698 ]
       },
       "no-comparison" : null,
       "succeeded" : null
    },
    "expected-results" : {
       "565/selftest1.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 8863920166200910451 ]
          ],
-         "ignore-failure" : true
+         "ignore-failure" : false
       },
       "8888/selftest1.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 13451349865803053525 ]
          ],
          "ignore-failure" : true
       }
    }
 }