Golang testing: b.StopTimer breaks b.Loop [1.24 backport]

@aclements requested issue #72922 to be considered for backport to the next 1.24 minor release.

@gopherbot, please backport to Go 1.24.

Comment From: cherrymui

Approving backport given that we do the simple fix mentioned in https://go-review.googlesource.com/c/go/+/658975/comment/6e3c2b44_92a7cc18/

Comment From: dmitshur

@JunyangShao Since you authored the original CL, would you like to create the cherry-pick CL for it too? The process is documented at https://go.dev/wiki/MinorReleases#making-cherry-pick-cls. Thanks.