lib/test: Add diagnostic Test.context for Mochawesome

This is, as far as I know, Mochawesome-specific, which makes it a bit
of a hack.  The Mochawesome side of this is
adamgruber/mochawesome@65e90621 (implement addContext method for
adding report context to tests, 2016-12-20,
adamgruber/mochawesome#106).  Ideally we'd want a way to trigger this
as part of requesting Mochawesome as the reporter, but the addition
seems harmless enough so I'm just including it every time.

In the diag.source case, we might want to expose diagnostics besides
.fn.  But we surely don't want to expose the test source via both .fn
and .context.  For this commit, I've left the diag.source case alone,
but in future work we might want something closer to :

  if (result.diag) {
    var context = Object.assign({}, result.diag)
    if (context.source) {
      var source = context.source
      delete context.source
      this.fn = {
        toString: function () {
          return 'function(){' + source + '\n}'
        }
      }
    }
    if (Object.keys(context).length) {
      this.context = {
        title: 'diagnostic',
        value: context,
      }
    }
  }

I haven't done that here, because Object.assign is not supported on
Internet Exporer, Android webview, or Opera for Android [1], despite
being part of ECMAScript 2015 [2].  Object.keys seems to be supported
everywhere [3].  We may only care about Node for this package, but I'm
being conservative for this commit.

[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#Browser_compatibility
[2]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#Specifications
[3]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys#Browser_compatibility

lib/test.js

@@ -13,11 +13,18 @@ function Test (result, suite) {
   } else {
     this.state = 'failed'
   }
-  if (result.diag && result.diag.source) {
-    var source = result.diag.source
-    this.fn = {
-      toString: function () {
-        return 'function(){' + source + '\n}'
+  if (result.diag) {
+    if (result.diag.source) {
+      var source = result.diag.source
+      this.fn = {
+        toString: function () {
+          return 'function(){' + source + '\n}'
+        }
+      }
+    } else {
+      this.context = {
+        title: 'diagnostic',
+        value: result.diag,
       }
     }
   }