As an anon user in an incognito window
Visit https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:MobileOptions
Check beta box
Click save.
Go back to settings page and you are still in stable
Seen in Chrome on desktop and nexus 5
When logged in this doesn't seem to happen.
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== Test Plan
* Visit https://en.m.wikipedia.beta.wmflabs.org/ as an anon.
* Visit the mobile settings page by opening the menu and clicking "Settings".
* Check "Beta".
* Click "Save".
* Observe that the `optin` cookie is set for the `.wikipedia.beta.wmflabs.org` domain.
* Visit the mobile settings page.
* Observe that "Beta" is checked.
* Switch to the desktop mode of the site by clicking "Desktop" in the footer menu.
* Observe that the `stopMobileRedirect` cookie is set for the `.wikipedia.beta.wmflabs.org` domain.
Repeat the above for your local wiki. If you're using MediaWiki-Vagrant, then you can visit both http://127.0.0.1:8080/ and http://dev.wiki.local.wmftest.net:8080, for which the cookie domains should be `127.0.0.1` and `.wiki.local.wmftest.net` respectively.