Add support for LUCI_CONTEXT.

recipe_modules/context/examples/full.expected/basic.json

 [
   {
     "cmd": [
       "bash",
       "-c",
       "echo default!"
     ],
     "name": "default step"
   },
   {
@@ skipping 95 matching lines @@
       "bash",
       "-c",
       "echo indent?"
     ],
     "name": "indented with wrong name",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]
   },
   {
+    "cmd": [
+      "echo",
+      "val"
+    ],
+    "name": "value in the context"
+  },
+  {
+    "cmd": [
+      "echo",
+      "zzz"
+    ],
+    "name": "value in the context (2)"
+  },

[iannucci, 2017/08/04 18:55:51]

the blacklist prevents the luci_config stuff from showing up here; I don't think
that's what we want.

+  {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]