Fixup rspec tag misuse

What does this MR do?

Specifies :mattermost as the sole tag within Test::Integration::Mattermost to fix test selection

Code introduced !14818 (merged) as part of gitlab-qa#26 (closed)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Hand testing of !14818 (merged) masked that we weren't specifying tags correctly.

https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/39154377 demonstrates the issue:

Run options: include {:core=>true, :mattermost=>true}

All examples were filtered out

Randomized with seed 28074

Top 0 slowest examples (0 seconds, 0.0% of total time):

Finished in 0.00181 seconds (files took 0.92329 seconds to load)
0 examples, 0 failures

We want at least the examples tagged :mattermost to run

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?