Robot tests: install our GS profile.

I noticed that on Jenkins only 142 robot tests were run instead of 149.  The console showed the reason:

```
  Set up plone.app.contenttypes.testing.PloneAppContenttypes in 0.020 seconds.
  Set up plone.app.robotframework.testing.SimplePublicationLayer in 0.001 seconds.
  Set up plone.app.robotframework.remote.RemoteLibraryBundle:RobotRemote in 0.000 seconds.
  Set up plone.testing.zope.WSGIServer in 0.043 seconds.
  Set up plone.app.robotframework.testing.RemoteLibrary:Robot in 0.000 seconds.
  Set up plone.app.discussion.testing.PloneAppDiscussionRobot Traceback (most recent call last):
  File "/Users/maurits/shared-eggs/cp311/zope.testrunner-6.4-py3.11.egg/zope/testrunner/runner.py", line 474, in run_layer
    setup_layer(options, layer, setup_layers)
  File "/Users/maurits/shared-eggs/cp311/zope.testrunner-6.4-py3.11.egg/zope/testrunner/runner.py", line 839, in setup_layer
    setup_layer(options, base, setup_layers)
  File "/Users/maurits/shared-eggs/cp311/zope.testrunner-6.4-py3.11.egg/zope/testrunner/runner.py", line 844, in setup_layer
    layer.setUp()
  File "/Users/maurits/shared-eggs/cp311/plone.app.testing-7.1.0-py3.11.egg/plone/app/testing/helpers.py", line 378, in setUp
    self.setUpPloneSite(portal)
  File "/Users/maurits/community/plone-coredev/6.1/src/plone.app.discussion/plone/app/discussion/testing.py", line 111, in setUpPloneSite
    settings = registry.forInterface(IDiscussionSettings)
  File "/Users/maurits/shared-eggs/cp311/plone.registry-2.0.1-py3.11.egg/plone/registry/registry.py", line 71, in forInterface
    raise KeyError(
KeyError: 'Interface `plone.app.discussion.interfaces.IDiscussionSettings` defines a field `globally_enabled`, for which there is no record.'
```

plone/app/discussion/testing.py

@@ -106,6 +106,7 @@ class PloneAppDiscussionRobot(PloneAppDiscussion):
     )
 
     def setUpPloneSite(self, portal):
+        applyProfile(portal, "plone.app.discussion:default")
         registry = queryUtility(IRegistry)
         settings = registry.forInterface(IDiscussionSettings)
         settings.globally_enabled = True