Fix error after an automatic incremental rebuild whilst running tests

When running tests in `gulp watch` mode, the initial run passed but
subsequent changes resulted in an error in the `merge-descriptors`
package, related to PhantomJS.

I haven't looked deeply into the problem but it does look like a
familiar issue related to Proxyquire trying to 'call through' to the
original imports in the module. Adding the '@noCallThru' flag in
guest-test.coffee resolves the issue.

h/static/scripts/annotator/test/guest-test.coffee

 Annotator = require('annotator')
 $ = Annotator.$
+Annotator['@noCallThru'] = true;
 
 highlighter = {}
 anchoring = {}