cmd/go: print test2json start events

cmd/go: print test2json start events

Add a new "Action":"start" test2json event to mark the
start of the test binary execution. This adds useful information
to the JSON traces, and it also lets programs watching test
execution see the order in which the tests are being run,
because we arrange for the starts to happen sequentially.

Change-Id: I9fc865a486a55a7e9315f8686f59a2aa06455884
Reviewed-on: https://go-review.googlesource.com/c/go/+/448357
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>