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, I'm just assuming that was already doing what
it was supposed to be doing, so this injection only affects cases
where diag is set but diag.source is not.

lib/test.js

@@ -9,11 +9,18 @@ function Test (result, parent) {
   this.title = result.name
   this.state = result.ok ? 'pass' : 'failed'
   this.pending = result.todo || false
-  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,
       }
     }
   }