cmd/go: add timestamps to script test output

cmd/go: add timestamps to script test output

Go tests don't include timestamps by default, but we would like to
have them in order to correlate builder failures with server and
network logs.

Since many of the Go tests with external network and service
dependencies are script tests for the 'go' command, logging timestamps
here adds a lot of logging value with one simple and very low-risk
change.

For #50541.
For #52490.
For #52545.
For #52851.

Change-Id: If3fa86deb4a216ec6a1abc4e6f4ee9b05030a729
Reviewed-on: https://go-review.googlesource.com/c/go/+/405714
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>