internal/testlog: use atomic.Pointer instead of atomic.Value
We know the type (*Interface), so we can use the generic atomic.Pointer.
This change also makes sure that concurrent use of SetLogger also
causes a panic, currently it races (Load, then Store).
Change-Id: I6fae5ce0587b37eede2060342c3fcd0cde4386ff
GitHub-Last-Rev:
0c053be03e22d4afcee235a247a377d7bd4d5aea
GitHub-Pull-Request: golang/go#69701
Reviewed-on: https://go-review.googlesource.com/c/go/+/616516
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>