Add GL_GROUP test case to the group API tests

Add GL_GROUP test case to the group API tests

Part of https://gitlab.com/gitlab-org/gitter/webapp/issues/2393

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The second commit fixes non-Promise returned from gitlab-backend causing an NPE.

Makes the backend-muxer more robust to handle non-Promise returns from downstream backends

Before when running the tests(npm run mocha -- test/request-api-tests/group-api-tests.js), I would see the following error

2020-01-21T06:29:20.727Z - error: Unable to send unread items notifications: TypeError: Cannot read property 'then' of null
{ exception:
   { message: 'Cannot read property \'then\' of null',
     stack:
      'TypeError: Cannot read property \'then\' of null
   at tryNext (modules\\backend-muxer\\lib\\backend-muxer.js:83:58)
   at modules\\backend-muxer\\lib\\backend-muxer.js:91:12
   at tryCatcher (node_modules\\bluebird\\js\\release\\util.js:16:23)
   at Promise._settlePromiseFromHandler (node_modules\\bluebird\\js\\release\\promise.js:512:31)
   at Promise._settlePromise (node_modules\\bluebird\\js\\release\\promise.js:569:18)\n
 at Promise._settlePromise0 (node_modules\\bluebird\\js\\release\\promise.js:614:10)
   at Promise._settlePromises (node_modules\\bluebird\\js\\release\\promise.js:693:18)
   at Async._drainQueue (node_modules\\bluebird\\js\\release\\async.js:133:16)
   at Async._drainQueues (node_modules\\bluebird\\js\\release\\async.js:143:10)
   at Immediate.Async.drainQueues [as _onImmediate] (node_modules\\bluebird\\js\\release\\async.js:17:14)\n
 at runCallback (timers.js:705:18)
   at tryOnImmediate (timers.js:676:5)
   at processImmediate (timers.js:658:5)
   at process.topLevelDomainCallback (domain.js:120:23)',
     name: 'TypeError' } }