gm: add --ignoreTests flag

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

Index: gm/tests/outputs/ignoring-one-test/output-expected/json-summary.txt
===================================================================
--- gm/tests/outputs/ignoring-one-test/output-expected/json-summary.txt	(revision 0)
+++ gm/tests/outputs/ignoring-one-test/output-expected/json-summary.txt	(working copy)
@@ -1,10 +1,10 @@
 {
    "actual-results" : {
-      "failed" : {
+      "failed" : null,
+      "failure-ignored" : {

[epoger, 2013/09/18 20:10:35]

The diff between patchsets 2 and 3 in this file shows us that the new flag is
being observed as expected.

          "565/selftest1.png" : [ "bitmap-64bitMD5", 12927999507540085554 ],
          "8888/selftest1.png" : [ "bitmap-64bitMD5", 1209453360120438698 ]
       },
-      "failure-ignored" : null,
       "no-comparison" : null,
       "succeeded" : null
    },
@@ -13,13 +13,13 @@
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 8863920166200910451 ]
          ],
-         "ignore-failure" : false
+         "ignore-failure" : true
       },
       "8888/selftest1.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 13451349865803053525 ]
          ],
-         "ignore-failure" : false
+         "ignore-failure" : true
       }
    }
 }