x/exp/trace: regenerate to include CL 691815 #76751
Description
This issue tracks updating golang.org/x/exp/trace to include the latest changes as of CL 691815. Normally this is as simple as running a script. https://go-review.googlesource.com/c/exp/+/724442 did this and ran into a few issues. There are a few changes needed which I feel warrant a little discussion:
- CL 710755 added the internal/trace/testtrace helper. This package depends on internal/testenv, which has a few transitive dependencies. Really the testtrace package only needs testenv.Builder right now, which is a one-line function that we can just copy by itself or inline. Are we okay with that?
- CL 691815 depends on internal/diff. That's currently self-contained enough that we could copy it into x/exp in its entirety, if we're okay with that.
- testtrace depends on testing.(*T).ArtifactDir, which is going to be in Go 1.26. Do we want to depend on Go 1.26 right away in the exported trace package? If not, should we replace the call with testing.(*T).TempDir or something else?