cmd/go: enable slog vet check during 'go test'

cmd/go: enable slog vet check during 'go test'

The slog check is new and no existing code uses slog (it's new too),
so there are no concerns about false positives in existing code.
Enable it by default.

Change-Id: I4fc1480eeb5a3acc9e5e095e9d5428f5ce04b121
Reviewed-on: https://go-review.googlesource.com/c/go/+/505915
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>