testing: within a B.Loop loop, assigning function result to _ allows body to be optimized away (1.26 regression) [1.25 backport]

[gopherbot]

@cherrymui requested issue #77654 to be considered for backport to the next 1.25 minor release.

@gopherbot please backport this to Go 1.26. This is a regression from 1.25. Thanks

[cherrymui]

1.25 works fine (with a different mechanism). No need to backport.