My apologies ahead of time if I'm overlooking the obvious, but I have not found any github or backdrop documentation that clarifies how to do this.
I am looking at an issue, #2351. I was involved in the discussion but never tested the pull request because I never did figure out how to do it in the short amount of time I had over the last couple weeks.
It shows, BWPanda added "status - has pull request" "pr - needs review" labels.
A followup comment said "I gave this a quick spin in the PR sandbox...".
So how does one do that?
I clicked on the "pr - needs review" link and it takes me to the entire list of issues needing review. So I search for issue #2351 using the filters field and it only finds issue #2402.
I went back and clicked on "status - has pull request". That takes me to the entire list of pull requests for all pr's. Again, a search for #2351, only finds issue #2402.
OK. Back to the original issue page, https://github.com/backdrop/backdrop-issues/issues/2351
I try clicking the "Pull requests" tab at the top of the page. That only reveals 1 open issue, #3409, which does not appear to be related to this issue at all. I click the "5 closed" link. That does not find this issue either.
On the https://backdropcms.org/contribute page, it says
we could use your help testing our work. Check the GitHub issue queue for issues that are tagged with needs review. There is a sandbox site created automatically for each PR, and we could use some help testing those sites for the intended changes.
But everything I try sends me either in circles or to dead ends.
I would think that the "has pull request" or the "pr - needs review" link would take me directly to the sandbox for testing. Or, at least, take me to the repository page with a link to download the code for testing.
After going in circles for a while, I finally noticed a link posted on the #2351 pr page to yet another pr, #2388. I clicked on that one, yet still no links anywhere to take me to a sandbox site for testing.