Fixup rspec tag misuse

In !14818 we added the ability to specify the example groups that should
run via the `-t` (tag) option to rspec.  It was assumed that the
inclusion filter generated `{:core => true, :mattermost => true}` would
run examples tagged :core OR :mattermost, when instead it means :core
AND :mattermost.

Here in order to assure at least some tests run, we specify that
the :mattermost tagged examples are the only ones we care about.

qa/qa/scenario/test/integration/mattermost.rb

@@ -7,7 +7,7 @@ module QA
         # including staging and on-premises installation.
         #
         class Mattermost < Scenario::Entrypoint
-          tags :core, :mattermost
+          tags :mattermost
 
           def perform(address, mattermost, *files)
             Runtime::Scenario.mattermost = mattermost