Fix slow + flaky namespaces_stat spec

[cyrillefr]

What this PR does

This PR aims to close #5660 by

• stubbing UpdateCourseStats to avoid real http calls
• building test course stats to get data
• removing now useless VCR

Open questions and concerns

I think it might closes #5655 as well

[cyrillefr]

Hello @ragesoss ,

I have got this Sentry related error:

(Sentry).capture_message("Non-200 response hitting references counter API", {:extra=>{:content=>{"description"=>"mwapi error: permissiondenied - You don't have permission to vie...e_code=>"en", :project_code=>"wikipedia", :rev_id=>708326238, :status_code=>403}, :level=>"warning"})

It happens because the code in lib/reference_counter_api.rb has been commented due to commit 6f40ec12b4

I guess it is better to make the line 29 it 'logs the message if response is not 200 OK' pending and I wait and rebase :)

[cyrillefr]

Better now after rebase :)

[ragesoss]

Looks good to me!