GM: include filename extension (.png) of each output file in JSON summary

gm/tests/outputs/compared-against-empty-dir/output-expected/json-summary.txt

Index: gm/tests/outputs/compared-against-empty-dir/output-expected/json-summary.txt
===================================================================
--- gm/tests/outputs/compared-against-empty-dir/output-expected/json-summary.txt	(revision 9118)
+++ gm/tests/outputs/compared-against-empty-dir/output-expected/json-summary.txt	(working copy)
@@ -3,21 +3,21 @@
       "failed" : null,
       "failure-ignored" : null,
       "no-comparison" : {
-         "565/selftest1" : {
+         "565/selftest1.png" : {
             "bitmap-64bitMD5" : 12927999507540085554
          },
-         "8888/selftest1" : {
+         "8888/selftest1.png" : {
             "bitmap-64bitMD5" : 1209453360120438698
          }
       },
       "succeeded" : null
    },
    "expected-results" : {
-      "565/selftest1" : {
+      "565/selftest1.png" : {
          "allowed-bitmap-64bitMD5s" : null,
          "ignore-failure" : false
       },
-      "8888/selftest1" : {
+      "8888/selftest1.png" : {
          "allowed-bitmap-64bitMD5s" : null,
          "ignore-failure" : false
       }